eM Client allow no subfolder from imported Outlook PST

Today I decided to replace MS Outlook 365 with eM Client. 
eM Client looks really good for me after watching some videos.

Installing and Setting works very easy and fast.
But there is a Problem to synch my Outlook Folders.

I ́d imported my backuped PST file into eM Client.
But there is a red exclamation Point behind every single imported subfolder.
After synch process there are hundreds of warnings (for each subfolder):
[IMAP] The Folder "[email protected] … couldn ́t be created on the Server. (Folder "INBOX/xxx/xxx allow no subfolder.)

Can anyone help me out with this?

GMX does not allow that depth of folders. On my GMX account I was only able to go 2 deep; so INBOX/xxx/xxx. Anything further is not allowed.

One option is to import the PST file into Local Folders, so they are stored on your computer. Then you can just keep them there, or change the folder structure before moving them to the GMX folders.