This is kind of weird. I decided to tinker with my signature some more. Not sure what's the cause of the 503 under certain circumstances, but I was able to modify the signature successfully with a workaround.... that is not consistent.
After a lot more tinkering, I tried out a lot of other permutations and am kind of getting mixed results. To spare you from reading a lot of it I think it boils down to how the links within the signatures are being handled.
For some reason any attempt at trying to replicate and isolate what I believe is a link issue has resulted in inconsistency.
The only thing that seems consistent in replicating a 503 is all or any partial deletions made to line 2 with the presence of line 1,
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This was a similar experience when I was trying to remove lines 3-5 in my old signature.
Also, the original workaround was replacing the entire signature with either nothing or a smiley, saving (successfully), and then simply pasting the item back in with the omissions. However, it's not working all the time. I was able to paste what was in the code section in a few times, but now I can't. Even though I'm replicating my previous workaround that had removed the content beneath line 2 previously.
Here's a clipping of the applied changes I was able to make and now I can't get to this result again,
Might be a long shot, but is there an escape character issue present? Or perhaps some bug involving the link feature within the signature module/component?
Kind of a hard topic to tackle since this doesn't seem to affect everyone and even though it may affect some it's not consistent.
Just trying to help shed some light from what I'm seeing on my end. Hope it helps. Let me know if you'd like me to try anything out. Also, feel free to abuse my signature if you'd like to use it for testing (not sure if it's account related, but I can't rule that out from my end).