Zapier Error (10)

Hi, I want to REPORT a BIG BUG. I almost got headache from this. However, I deleted some custom fields I never use and then Zapier suddenly stopped working and give me those errors. So I thought I reassign the custom fields 1 by 1 and then it worked again (look screenshot). The Error (10) is happening.

So I can’t delete my custom fields forever? This is kind of … hmm

Only thing I can think of there is that maybe if any of those fields are used in another zap?