Error synchronizing with Google Calendar

I have my Google Calendar added through the network/gmail account process. I can see the events from my Google cal in TBird. When I accept an invite that came into my email in TB, it gets added to the TB calendar but never syncs back to my Google cal. Looking at the Error Console, I keep getting "Server Replied with 500"' errors (full error code is below). I already tried all the online steps - delete/add calendar, clear password, reset OAuth token, still get the same issue. calendar: [calCachedCalendar] Unable to perform playback action add to the server, will try again next time (longnumbersequence,[Exception... "Server Replied with 500" nsresult: "0x80040111 (NS_ERROR_NOT_AVAILABLE)" location: "JS frame :: resource:///modules/CalDavCalendar.sys.mjs :: onOperationComplete :: line 580" data: no]) CalCachedCalendar.sys.mjs:570:20