You receive error message Service application unavailable on website.

When checking IIS log, you see error Application pool is beiing automatically disabled due to a series of failures in the process(es) serving that application.

The error means there is some bug in website code crashing IIS application with too many errors in a short period of time.

As a temporary solution, you can disable Rapid fail protection in Application pool setting

IIS manager > Application pools > select application pool > advanced settings

You will need to review and debug website to find out which piece of code causing problem.