PST Migration Tool: Import not working correctly and max Export size 2GB

Hello everyone,

I want to migrate about 15 mailboxes from one O365 tenand to another existing tenand.

Export:

The export of the mailboxes worked fine on my machine.
On several other machines (also Servers and VMs) the file size maxed out at 2GB. (I tried different things but I can only get it to work on my computer, not on all others. I even tried a new Windows Installation, same result.)

Import:

Now my other problem:

After I import a mailbox, the application tells me "Successfully imported mailboxes" but I can't see any mail in the target mailbox.

import.log: (changed domain and user)

23.09.2019 11:38:16 [FINE] : ExchnageOnlineImporter invoked
23.09.2019 11:38:16 [FINE] : Import Triggered...
23.09.2019 11:38:16 [INFO] : Thread-Id : 13 : Logging to mailbox using ews
23.09.2019 11:38:16 [INFO] : Thread-Id : 13 : Mailbox login success
23.09.2019 11:38:16 [FINE] : Thread Limit : 3
23.09.2019 11:38:16 [FINE] : Required Threads : 1
23.09.2019 11:38:16 [FINE] : No. of Threads created : 1
23.09.2019 11:38:16 [INFO] : Thread-Id : 27 : Import for tamara.tester@***.de.pst started
23.09.2019 11:38:16 [INFO] : Thread-Id : 27 : Creating instance for pstSession Object
23.09.2019 11:38:16 [FINE] : Thread-Id : 27 : C:\Users\myuser\Desktop\PST\tamara.tester@***.de.pst log in success
23.09.2019 11:38:16 [FINE] : Thread-Id : 27 : Requesting access for tamara.tester@***.de
23.09.2019 11:38:17 [FINE] : Thread-Id : 27 : Access to tamara.tester@***.de provided
23.09.2019 11:38:18 [INFO] : System Error. Code: 18.
Es sind keine weiteren Dateien vorhanden
23.09.2019 11:38:18 [INFO] :    bei Redemption.IRDOMail.get_ReceivedTime()
   bei PST_Tool.ExchnageOnlineImporter.copyItems(RDOFolder source, Folder destination, String emlFilePath)
23.09.2019 11:38:18 [INFO] : System Error. Code: 18.
Es sind keine weiteren Dateien vorhanden
23.09.2019 11:38:18 [INFO] :    bei Redemption.IRDOMail.get_ReceivedTime()
   bei PST_Tool.ExchnageOnlineImporter.copyItems(RDOFolder source, Folder destination, String emlFilePath)
23.09.2019 11:38:26 [INFO] : System Error. Code: 18.
Es sind keine weiteren Dateien vorhanden
23.09.2019 11:38:26 [INFO] :    bei Redemption.IRDOContactItem.get_ReceivedTime()
   bei PST_Tool.ExchnageOnlineImporter.copyItems(RDOFolder source, Folder destination, String emlFilePath)
23.09.2019 11:38:30 [INFO] : Thread-Id : 27 : C:\Users\myuser\Desktop\PST\tamara.tester@***.de.pst imported successfully