Thunderbird does not allow to save calendar invites to 1/2 network calendars anymore

Hello,

I have used Thunderbird + lightning for quite a long time. Some time ago, I set up Provider for Exchange ActiveSync & TbSync.

I have two Outlook accounts and one calendar for each account. When calendar invitations are sent to me, I used to be able to "accept" them and subsequently add them to either of the two calendars of my choosing.

However, I recently started noticing that when I want to add an event to a calendar, only one of the calendars shows up!

I've also started to notice some other funky Thunderbird activities, like *some* email accounts not checking for new messages.

Where can I start to sort out this problem I'm running Fedora 30 and using Thunderbird 60.8.0 (64-bit), with the following extensions:

Attach from Clipboard
AttachmentExtractor Continued
Awesome Auto Archive
Display Mail User Agent
FiltaQuilla
Lightning
Lightning Calendar Tabs
Lightning Invitation Notifier
Mail Merge
Mail Redirect
Mail Summaries
MailHops
Manually sort folders
MinimizeToTray Reanimated
Provider for Exchange ActiveSync
Remove Duplicate Messages (Alternate)
Search Results Sort By ate Not Relevance
Send Later
TbSync
Thunderbird Conversations
ThunderHTMLedit
XNote+++

Thanks for any help!!