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It seems that a helper function for generate query strings in my functions.php cause this issue:
Perhaps you could output the migration process with a status list of the processed posts? In this case, the error was relatively easy to find because the error text already clearly indicated the location. But there was nothing to be found in the debug.log.
It would be sufficient to also write this list in a log file so that you have an overview of which pages have already been migrated and which are causing problems.