I can post at times on browsers that have their own VPNs. I use a commercial VPN that won't post on clearnet, but if I turn on the browser VPN on top of that I can post. That would mean inability to post/post images isn't browser/OS related.
Have to figure spam mitigation is blocking offending IP ranges from write access. IPs are geographically assigned, so if you are in that geographical area you'll be caught up in the block. Remedy is to find a VPN node that uses an unblocked IP, IMO.
Using the bypass link helps.
https://sys.8kun.top/dnsbls_bypass.php
You may have to do that every post, depending on how the VPN is routing your request.
I've found, though, that I can't even access that bypass link with certain VPNs. Adds to my suspicion that 8Kun is blocking IP ranges to stop the bleeding while a permanent fix is being implemented.