Client v. 7 with Server v. 9

I have a Seafile Installation (v. 9.0.6) on a Raspberry and so far everything works well across multiple devices.
However, I have now a laptop running Windows 8 that has Seafile Client v. 7.0.10, which is the latest version that does not require Windows 10.
On said laptop, I experience frequent upload errors (ccnet/logs/seafile.log):

Repo ‘Files’ sync state transition from uploading to ‘error’: ‘Data transfer was interrupted. Please check network or firewall’.

These errors are random. Some file in some libraries work fine, others not at all. I tried unsynching and resynching and even completely recreating the library. In one instance, I simple let it run over night, the next day everything was unsynchronized.

I found no logic in what files can be uploaded and what files can’t. I tried restarting my laptop, making sure no other program is currently accessing those files, nothing seems to work.

Since I have no similar errors on any machine I suspect a problem with the client version, since it is rather old. However, it is impossible for me to upgrade it.

Any tips on how I can make it work? From my point of view, a server update should not require an operation system update on the client, but also I have no way of verifying if this is indeed the problem.