Tried dozens of times just clicking "save" repeatedly, never fails:
X3 will not just randomly fail with this ... If it returns parse error, it means the returns script does not load, which in 100% of cases I have dealt with, is because of server Firewall. The firewall could be triggering based on something specific you are adding to panel, but then I need to be able to reproduce it. This is not an issue native to X3 ... how could we make it fail randomly?
If you can't find out exactly WHEN it happens, so I can try to reproduce it, then the best I can do is ask a few others to visit your panel and try to save, in an attempt to reproduce a FAIL.