I need some help. I have deleted much of my IMAP inbox erroneously by setting the retention date in Kmail to 1 day*.

Now I had these mails locally in K-Mail and eant to export them back to my thunderbird installation (.eml or .sbd /mbox files)
Error: The exported ZIP file is empty. Savin is finished after a second, and it contains only a few kilobytes.
Onscreen messages show only inclusion of settings and parameters, no e.mails

I used the built-in export tool, and closed KMail before each attempt.
It seems the export tool exports everything EXCEPT e-mails. If anything else except for e-mails is disabled for export, it just exports nothing at all.

*This was an attempt to overcome another persistent problem with KMail, not realizing this also refers to the IMAP inbox
I got an error message every time I try to delete mails/a folder: "E-mails cannot be downloaded due to missing storage space"
--> KMail is totally stuck.
I even uninstalled it and re-installed -> no effect, even all the mails are still there ...
Kmail is installed on a Kubuntu OS within a Virtualbox VM. Virtual disk size is dynamic and has room for another 10 GByte (I recently resized it from 10 to 20 Gbytes).