Hi Micknet,
When importing, please choose “Import items into the current folder” in the following dialog box to see if it works.

If error continues, instead of importing, please try opening this PST file in Outlook. To do this, click File > Open&Export > Open Outlook Data File.
If it opens normally, we can manually copy/move folders to your Exchange account as a workaround.
Hope the information is helpful to you.
Regards,
Perry
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