Just as a further update to those seeking answers:
I have been getting this error since the day it was reported. I have been trying varying amounts a couple of times a day since. Errors have continued.
I saw the post above that 1000L sells were going through, so today I tried:
1000L - Sold
5000L - Sold
10,000L - Error
6000 - Error
1000 - Error
I do not know if it would have worked had I stayed at 5k or if it would have errored again.
I do not believe using a VPN or restarting your router will have any affect on this at all.
From what I've seen posted on SL Facebook, Process Credits have been going through. This is a sell issue.