I guess you mean this article:

https://dt.picbasic.co.uk/INT16/Customisation

I am pretty sure that did something stupid and the modified interrupt did not work as expected. Ended to make a poll of the interrupt bit and finished my project.

OK, will give it a try again. With a bottle of coffee this time!

Ioannis

P.S. The 18 series requires modifications in 2 places. Why is that? Because of the priorities?