E-Mails are cleared out from IMAP-folders after mailbox migration

The background:
I own a private domain and use the service of a provider for E-Mail servic=
es=20
to that domain. Because my long term provider decided to impose a migratio=
n=20
from IMAP/SMTP to Microsoft356 upon its customers, I had to change my=20
provider. I decided to do that by making a local copy of all my E-Mails in=
 my=20
inbox and all IMAP Folders below and then copy them back to an IMAP/SMTP=
=20
account which was set up freshly at my new provider. All this should by do=
ne=20
by Kontact/KMail.

System: kdepim/akonadi binary packages of a standard OpenSuse 15.6=20
installation, version: 23.08.5, release bp156.1.3.

After sucessfully setting up the fresh account I moved E-Mails and folders=
=20
back to the Inbox. Then the first weird thing occurred: Some of those fold=
ers=20
reappeared in the local folder structure. I removed those obvious duplicat=
es=20
by deleting the folders one by one in the tree view.

After a few minutes I noticed that some of the folders in the IMAP tree we=
re=20
cleared out (not all!). Fortunately I had a backup on an external harddisk=
, so=20
I was able to restore the missing E-Mails in a local tree (although it too=
k=20
quite some time). However, after copying the E-Mails from their local to t=
he=20
corresponding IMAP folder the same happened again. The IMAP folders were=
=20
emptied again after a few minutes.

I repeated this several times, always the same. I can confirm that the E-M=
ails=20
were copied to the server of the provider, I checked this via Webmail. So=
=20
there must be something in the client/agent/akonadi chain that makes that=
=20
undemanded folder clearence. =20

Please give me some advice, how to solve that problem.

Best regards
Thomas Hejze