Here is the stack from RESTier, which suggests that the issue is in the UriParser within OData Library (version ODL 7.6.1):

at IEdmTypeReference Microsoft.OData.UriParser.FunctionCallBinder.ValidateIsOfOrCast(BindingState state, bool isCast, ref List args)\r\n at SingleValueNode Microsoft.OData.UriParser.FunctionCallBinder.CreateUnboundFunctionNode(string functionCallTokenName, List args)\r\n at QueryNode Microsoft.OData.UriParser.FunctionCallBinder.BindAsUriFunction(FunctionCallToken functionCallToken, List argumentNodes)\r\n at QueryNode Microsoft.OData.UriParser.FunctionCallBinder.BindFunctionCall(FunctionCallToken functionCallToken)\r\n at QueryNode Microsoft.OData.UriParser.MetadataBinder.BindFunctionCall(FunctionCallToken functionCallToken)\r\n at QueryNode Microsoft.OData.UriParser.MetadataBinder.Bind(QueryToken token)\r\n at FilterClause Microsoft.OData.UriParser.FilterBinder.BindFilter(QueryToken filter)\r\n at FilterClause Microsoft.OData.UriParser.ODataQueryOptionParser.ParseFilterImplementation(string filter, ODataUriParserConfiguration configuration, ODataPathInfo odataPathInfo)\r\n at FilterClause Microsoft.OData.UriParser.ODataQueryOptionParser.ParseFilter()\r\n at FilterClause Microsoft.AspNet.OData.Query.FilterQueryOption.get_FilterClause()\r\n at void Microsoft.AspNet.OData.Query.Validators.FilterQueryValidator.Validate(FilterQueryOption filterQueryOption, ODataValidationSettings settings)\r\n at void Microsoft.AspNet.OData.Query.FilterQueryOption.Validate(ODataValidationSettings validationSettings)\r\n at void Microsoft.AspNet.OData.Query.Validators.ODataQueryValidator.Validate(ODataQueryOptions options, ODataValidationSettings validationSettings)\r\n at void Microsoft.AspNet.OData.Query.ODataQueryOptions.Validate(ODataValidationSettings validationSettings)\r\n at async Task<(IQueryable Queryable, ETag Etag)> Microsoft.Restier.AspNet.RestierController.ApplyQueryOptionsAsync(IQueryable queryable, ODataPath path, bool applyCount)\r\n at async Task Microsoft.Restier.AspNet.RestierController.Get(CancellationToken cancellationToken)\r\n at async Task System.Threading.Tasks.TaskHelpersExtensions.CastToObject(Task task)\r\n at async Task System.Web.Http.Controllers.ApiControllerActionInvoker.InvokeActionAsyncCore(HttpActionContext actionContext, CancellationToken cancellationToken)\r\n at async Task System.Web.Http.Controllers.ActionFilterResult.ExecuteAsync(CancellationToken cancellationToken)\r\n at async Task System.Web.Http.Controllers.ExceptionFilterResult.ExecuteAsync(CancellationToken cancellationToken)

+1, encountered this issue today