Subfolders in Mail App not always available

I have 4 Mail Accounts in my Mail App V 1.0.0 running on Nextcloud 17.0.2. One GMail one hotmail and two t-online. But for some reason the subfolders I have in my t-online accounts don’t show up. When I click on show all folders nothing happens for these two accounts. It works for the other two tho. I never had a problem with my T-online Accounts. Those folders are available in all other E-Mail Clients I use. Has anyone an idea what could be the reason for this?