No or Partial Sync on Shared Lists

Hello

We use a Shared List at work (5 People). We keep having problems with it:
No/Partial Syncing, Solution varies from logging off, leaving list etc. (we have had this problem with nearly all users).

However now my Boss (who is part of the list too) got a new (Win10) Laptop as his main device, his problems began with his desktop app not Syncing anything (everything empty), however we could fix that by logging off and logging back in. Now we have the problem that lists wont sync, e.g. he adds a task for me but it wont show up etc.

I told him to leave the list, BUT: now when he wants to rejoin by me sending an invite link it says that the list is not available.

I have thought that the problem might be that we are using To Do quite a while already and that all the many and recent updates have rendered our old list "incompatible" (?).

Hope that someone has a fix.

regards