I've seen this before, but unfortunately I can't remember exactly what the fix was. I do remember it being something annoyingly simple though, once I figured it out anyway.

I believe it was something like the field having a default value set, as well as being filled by a lookup, so it wasn't sure which to use? I could be way off though.

How many read-only fields are on your form? It might be worth going through them one by one and seeing if they have default values, or calculations, or anything of the sort that might be contradicting your lookup rule.

Editing to add: I went back through my old messages and confirmed that I did have the same error, and the issue was having both a default value set and a value being pulled from a lookup. Hopefully it's the same in your case!

Sorry, one more edit: Re-reading my messages it might have been a default value set on a field variable that was filled in earlier in the process. So, same idea in that it was trying to overwrite itself, but not necessarily from a lookup table.