While searching for a solution for this issue I found out, that after fixing 'phpBB/includes/functions_upload.php' the upload of larger files containing umlauts in theier filenames fails.
I used a file called 'Test-üäa.docx' with about 4 MiB file size with embedded pictures. A smaller file (ca. 50 KiB) 'Test-ü.docx' uploaded without problems.

{"jsonrpc":"2.0","id":"id","error":{"code":105,"message":"Test-u??a??o??.docx ist ein ung\u00fcltiger Dateiname."}}