I have the same issue with Community edition (as described) and just found out that I have to switch to cache mode (and it is still synchronizing as I have large email accounts with >30GB). But in fact: this is not my preferd solution.
There is nothing wrong with my email data, just Outlook 365 is not working as expected with Grommunio in Online-Mode.
What I found out, that this is not the case for all emails (for instance: yesterday there was one, which was readable), but for the most received in the last months…