Folder sync no longer works for T-Online Mails

I've been using the Thunderbird App (v12.1) on a Google Pixel 8 (Android 16) for a while now with several Email accounts from different providers. One of them is a T-Online Email address. Recently it appears to have stopped syncing the folders only for this account. I still get emails in the inbox but all the folders are empty. It only says "Bis zu 100 weitere abrufen" (request up to 100 more) but tapping that doesn't make a difference. In the settings I have the sync set to everything without limitation. I tested the same account with K9 (v11.1) and Google Mail and in both all the folders have their content. Does anyone have the same issue or any suggestions? Is there any logging that I turn on to see if there's any errors? Thanks!