Damp in bottom corner of UPVC door aperture

[AddyMont]

Hi, I'm new on here trying to get some help with a problem that's come up in our house.

We moved in October 2022 and a damp patch has come up in the bottom right corner of our backdoor aperture, there is discoloration and the plaster has blown.

Looking from outside the mortar has crumbled by the sill and it looks as if the previous owner has filled the gap with foam.

My uneducated thoughts are that being above the damp course, moisture could be gathering on the flat surface and permiating through the wall.

Any opinions on what could be causing it and suggestions on how to fix the problem?

Thanks in anticipation, Addy.

 

[DiyNutJob]

Zoom out on the exterior pics to give a better context. If there is an opening somewhere, then water will get in