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.