This is such called "и - е swap" (мена и - е)
Trubachyov in Труды по этимологии: Слово. История. Культура attributes it to Russian glycolalia (сладкоязычие) which tends to mix stressed й and ль in some dialects.
It is common to replace stressed ий with ей in Russian. That's why we have names like Сергей, Мокей, Алексей (from Сергий, Мокий, Алексий), words like змей, книгочей and the dialectal word Расея.
Glycolalia, among other things, makes stressed иль to be replaced with ель in some words by analogy with ей < ий:
- апрель < априль < Aprilis
- канитель < канетиль < cannetille
This contamination is most probably the cause of the swap in the word you're mentioning:
- плательщик (= платейщик) < платильщик (= платийщик)
- молельщик (= молейщик) < молильщик (= молийщик)