Update : We have now succeeded in upgrading both of the 18.0.21 servers to 18.0.24 (the sources at our provider OVH were faulty and not allowing update beyond 18.0.21).
We have also succeeded in updating the Onxy server to Obsidian 18.0.24. We still cannot migrate anything from the old server to...
We have a dying server (running Plesk Onyx 17.8.11 #82) and a new server to replace it running Obsidian 18.0.21.
Trying to migrate from the old server to the new one produces the above error.
I have Googled and tried a couple of suggestions but none of them have fixed the issue...and there isn't...