I cannot be of much help unfortunately. Only thing I can tell you is that the fix regarding libsocialcache is already integrated but the latest version of buteo-sync-plugins-social is not.
The second package seems to be at 0.4.27 on my C2 (updated to 5.0.0.72) while there are versions 0.4.28 and 0.4.29 available on Tags · sailfishos/buteo-sync-plugins-social · GitHub . You may try to build the rpm and install on your device to check if that helps…