Buster+PSB screen flashes like crazy

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