Cannot downgrade to xpra=5.0.7-r0-1 (focal)

[stdedos]

xpra nowadays is a meta-package, strictly depending (=) on other packages, for modularity.

I'd like to downgrade it (from 5.0.8-r0-1) for testing purposes. Probably doing that on apt-get would get a lot tedious, so I wanted to try a GUI.

Sadly, I am getting this error:

Failed to SetCandidateRelease for focal

synaptic/common/rpackage.cc

Lines 1198 to 1200 in 9885158

	if(!_depcache->SetCandidateRelease(Ver, archive))
	std::cerr << "Failed to SetCandidateRelease for " << archive << std::endl;
	break;

I know I am on an old system, but the code looks awfully similar.
Even if this is not / cannot be fixed, I thought an FYI is nice-to-have.