Content of Spam folder is not shown


I have been using K-9 for nine years now.
I use version 5.600 under Android 7.1 on a Sony Xperia Z5 Premium with imap4.
Until two weeks ago I could see email having been filtered to the folder Spam by my IMAP4 server at Ionos.
Then I got a notification about an email that was filtered to the folder Spam by Ionos.
I synced my Spam folder via K-9, but it appears empty.
K-9 states it is synced.
But when I look at my Spam folder via a webclient, Thunderbird or Pegasus Mail I can see all the emails in the folder Spam.
Only K-9 seems to hide its content.
This problem didn’t happen before two weeks!
I phoned Ionos, but they told me that they didn’t change anything and it must be a problem with K-9.
How can I solve this, so that I can see the emails in the folder Spam again?
What might be the problem here?
It doesn’t occur with my other accounts e.g. GMX in my K-9!

The 5.6 version of K-9 hasn’t been updated in some time so a recent change in behavior can’t be attributed to the client proper.

Did you happen to change the folder classes that are synced, or the folder class of your “spam” folder for this account? I don’t have 5.6 handy, but I believe you will find the “poll folders” option in the “fetching mail” settings. Make certain that the folders you want synced are in the classes that you sync.

I do know, that my version of K-9 is from September 2019 and nothing has been changed since then.

Therefore I supposed, that my provider had changed something.

But they refused having changed anything.

I didn´t change anything in my K-9 settings.

So, I have no idea what might have happened.

Three weeks ago I could see emails in my Spam folder, now they are kind of hidden but only in K-9.

But I can see them in Thunderbird, Pegasus and the webmailer.

Strange is that the problem only occurs with this one mailaccount.

Spam folders of other mail accounts are still shown in my K-9.

Maybe something in my K-9 has accidently been destroyed?

I would like to delete this Spam folder in my K-9 and then reinstall it.

I tried this, but couldn´t find any option in the settings to delete a folder, which is already in the list.

Reinstalling will be easy, because I just manually have to search my IMAP folders.

So, can anybody help me solving this strange problem?