I am seeing the same issue also at this point I can not even use the web mail interface.
I have multiple computers with outlook same account all set to offline cache and if I only use them the sync works fine.
I can view the emails on the web and see all emails in the folders I changed via outlook.
Were things break is the moment I make any changes even deleting an email the sync messages start flowing.
Where I first noticed it is in sent items I had did on the web were not in my outlook.
Another thing that happens is once the sync error occurs even is you delete the account or clear the cache for the offline folder outlook will not sync any of the mail into the folder only new items even if they are all on the server.
It seems like all the folders except the inbox seen to sync.