Just upgraded my laptop from Stretch+R14 to Buster+PSB, after several 
successful test VPS upgrades.

After logging in screen flashes like crazy and .xsession-errors
mostly repeats (with different resource ids):

X Error: BadWindow (invalid Window parameter) 3
  Major opcode:  3
  Minor opcode:  0
  Resource id:  0x390a155

Also seeing a few of these:

 Error: BadAccess (attempt to access private resource denied) 10
  Major opcode:  2
  Minor opcode:  0
  Resource id:  0x180001c

And an occasional:

unix_connect: can't connect to server 
(unix:/tmp/ksocket-mgb/bravo.y.n-262a-51c100e0)
[artsd] There are already artsd objects registered, looking if they are 
active...
[artsd] ... cleaned 5 unused mcop global references.

However login screen is fine and I can login in Failsafe mode and
then run firefox and kmail from konsole without problems.

Any suggestions or anything I can do to help diagnose/debug problem?

TIA,

--Mike