Thank you Syam. In the meantime, this is what we did which could also give you more information:

WITH WPFront User Role Editor activated:
– wp ajax request with jquery old ajax function doesn’t work, retrieve 500 error.
– wp ajax request with jquery new ajax function andphp limit memory to 1GB works but slow. (it was 512 MB)

WPFront User Role Editor deactivated
– wp ajax request (with jquery old ajax function) and normal php limit memory of 512MB work as normal.