What could have gone wrong with my TLP222A?
What is the load you switch?


  It's actually just a simple LED (it just accepts industrial voltage -
  24V)


No it isn't. 

It's an LED module that can run from AC or DC and has in all probability, a bridge rectifier and smoothing capacitor that has an inrush current. The data sheet states this: -


  Maximum inrush current max. 500 mA


That is right at the limit of your TLP222A's capabilities and will seriously degrade the life of that product. Toshiba say this: -


  Note: Using continuously under heavy loads (e.g. the application of
  high temperature/current/voltage and the significant change in
  temperature, etc.) may cause this product to decrease in the
  reliability significantly even if the operating conditions (i.e.
  operating temperature/current/voltage, etc.) are within the absolute
  maximum ratings.