Dear Elias.
ACP > Customise > Manage Extensions is not the problem.
It exists, and an extension can be seen there when uploaded to the /forum/ext folder.
But when trying to install the extension, I get the error described above and no extension will be installed, only this error appears and no EXT tab appears either.
Which should appear after installing an extension, as seen in other phpbb forums I administer.
But no extensions will be installed due to this error.