diff --git a/util-networks.R b/util-networks.R
index 648fbac9..1edbb4ab 100644
--- a/util-networks.R
+++ b/util-networks.R
@@ -978,6 +978,7 @@ NetworkBuilder = R6::R6Class("NetworkBuilder",
                 attr(net, "range") = private$proj.data$get.range()
             }
 
+            net = convert.edge.attributes.to.list(net)
             return(net)
         },
 
@@ -1026,6 +1027,7 @@ NetworkBuilder = R6::R6Class("NetworkBuilder",
                 attr(net, "range") = private$proj.data$get.range()
             }
 
+            net = convert.edge.attributes.to.list(net)
             return(net)
         },
 
@@ -1068,6 +1070,7 @@ NetworkBuilder = R6::R6Class("NetworkBuilder",
                 attr(net, "range") = private$proj.data$get.range()
             }
 
+            net = convert.edge.attributes.to.list(net)
             return(net)
         },
 
@@ -1174,6 +1177,7 @@ NetworkBuilder = R6::R6Class("NetworkBuilder",
                 attr(network, "range") = private$proj.data$get.range()
             }
 
+            network = convert.edge.attributes.to.list(network)
             return(network)
         },
 
@@ -1285,15 +1289,15 @@ NetworkBuilder = R6::R6Class("NetworkBuilder",
             ## Note: The following temporary fix only considers the 'date' attribute. However, this problem could also
             ## affect several other attributes, whose classes are not adjusted in our temporary fix.
             ## The following code block should be redundant as soon as igraph has fixed their bug.
-            u.actual.edge.attribute.date = igraph::edge_attr(u, "date")
-            if (!is.null(u.actual.edge.attribute.date)) {
-                if (is.list(u.actual.edge.attribute.date)) {
-                    u.expected.edge.attribute.date = lapply(u.actual.edge.attribute.date, get.date.from.unix.timestamp)
-                } else {
-                    u.expected.edge.attribute.date = get.date.from.unix.timestamp(u.actual.edge.attribute.date)
-                }
-                u = igraph::set_edge_attr(u, "date", value = u.expected.edge.attribute.date)
-            }
+            # u.actual.edge.attribute.date = do.call(base::c, igraph::edge_attr(u, "date"))
+            # if (!is.null(u.actual.edge.attribute.date)) {
+            #     if (is.list(u.actual.edge.attribute.date)) {
+            #         u.expected.edge.attribute.date = lapply(u.actual.edge.attribute.date, get.date.from.unix.timestamp)
+            #     } else {
+            #         u.expected.edge.attribute.date = get.date.from.unix.timestamp(u.actual.edge.attribute.date)
+            #     }
+            #     u = igraph::set_edge_attr(u, "date", value = as.list(u.actual.edge.attribute.date))
+            # }
 
             ## 2) add the bipartite edges
             u = add.edges.for.bipartite.relation(u, authors.to.artifacts, private$network.conf)
@@ -1632,6 +1636,7 @@ construct.network.from.edge.list = function(vertices, edge.list, network.conf, d
 
     ## initialize edge weights
     net = igraph::set_edge_attr(net, "weight", value = 1)
+    net = convert.edge.attributes.to.list(net)
 
     logging::logdebug("construct.network.from.edge.list: finished.")
 
@@ -1792,6 +1797,13 @@ add.edges.for.bipartite.relation = function(net, bipartite.relations, network.co
         extra.edge.attributes["type"] = TYPE.EDGES.INTER # add egde type
         extra.edge.attributes["relation"] = relation # add relation type
 
+        ## convert edge attributes to list similarely to 'convert.edge.attributes.to.list'
+        edge.attrs = names(extra.edge.attributes)
+        which.attrs = !(edge.attrs %in% names(EDGE.ATTR.HANDLING))
+        for (attr in edge.attrs[which.attrs]) {
+            extra.edge.attributes[[attr]] = as.list(extra.edge.attributes[[attr]])
+        }
+
         ## add the vertex sequences as edges to the network
         net = igraph::add_edges(net, unlist(vertex.sequence.for.edges), attr = extra.edge.attributes)
     }
@@ -1812,7 +1824,7 @@ create.empty.network = function(directed = TRUE, add.attributes = FALSE) {
     # set proper attributes if wanted
     if (add.attributes) {
         mandatory.edge.attributes.classes = list(
-            date = c("POSIXct", "POSIXt"), artifact.type = "character", weight = "numeric",
+            date = "list", artifact.type = "list", weight = "numeric",
             type = "character", relation = "character"
         )
         mandatory.vertex.attributes.classes = list(name = "character", kind = "character", type = "character")
@@ -2146,6 +2158,34 @@ get.data.sources.from.relations = function(network) {
     return(data.sources)
 }
 
+#' Convert edge attributes to list type.
+#'
+#' This conversion is necessary to ensure merging networks works in all cases,
+#' especially when merging simplified networks with unsimplified networks as
+#' simplification may convert edge attributes to list type. Attributes that are
+#' explicitly considered during simplification (through EDGE.ATTR.HANDLING)
+#' generally do not need to be converted.
+#'
+#' @param network the network of which the edge attributes are to be converted
+#' @param remain.as.is the edge attributes to remain as they are
+#'                     [default: names(EDGE.ATTR.HANDLING)]
+#'
+#' @return the network with converted edge attributes
+convert.edge.attributes.to.list = function(network, remain.as.is = names(EDGE.ATTR.HANDLING)) {
+
+    ## get edge attributes
+    edge.attrs = igraph::edge_attr_names(network)
+    which.attrs = !(edge.attrs %in% remain.as.is)
+
+    ## convert edge attributes to list type
+    for (attr in edge.attrs[which.attrs]) {
+        list.attr = as.list(igraph::edge_attr(network, attr))
+        network = igraph::set_edge_attr(network, attr, value = list.attr)
+    }
+
+    return(network)
+}
+
 
 ## / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / /
 ## Sample network ----------------------------------------------------------
diff --git a/util-split.R b/util-split.R
index 2bce7739..bb51e517 100644
--- a/util-split.R
+++ b/util-split.R
@@ -524,7 +524,7 @@ split.network.time.based = function(network, time.period = "3 months", bins = NU
                                     number.windows = NULL,  sliding.window = FALSE,
                                     remove.isolates = TRUE) {
     ## extract date attributes from edges
-    dates = get.date.from.unix.timestamp(igraph::edge_attr(network, "date"))
+    dates = do.call(base::c, igraph::edge_attr(network, "date"))
 
     ## number of windows given (ignoring time period and bins)
     if (!is.null(number.windows)) {
@@ -619,7 +619,7 @@ split.networks.time.based = function(networks, time.period = "3 months", bins =
             dates = igraph::E(net)$date
             return(dates)
         })
-        dates = unlist(networks.dates, recursive = FALSE)
+        dates = unlist(networks.dates)
         dates = get.date.from.unix.timestamp(dates)
 
         ## 2) get bin information
@@ -709,7 +709,7 @@ split.network.activity.based = function(network, number.edges = 5000, number.win
 
     ## get dates in a data.frame for splitting purposes
     df = data.frame(
-        date = get.date.from.unix.timestamp(igraph::edge_attr(network, "date")),
+        date = do.call(base::c, igraph::edge_attr(network, "date")),
         my.unique.id = seq_len(edge.count) # as a unique identifier only
     )
     ## sort by date