PHP max_execution_time is now 330
PHP memory_limit is now 512 (maximum setting)
PHP file_get_contents(): as far as I know, this is a standard function of PHP which is always avaiulable.
I also sked my ISP whether they could switch off Modsecurity; but this can only be done at server level. The can switch off specific rules at domain level.
I appreciate your help very much. But I do not understand why SPPB 5 has such very extreme requirements which were not necessary for SPPB 3 and not for any other plugin that I am using, includign Joomla itself.
Edit
Things are getting nasty. When I open my front page in teh backend editor (made in the past with SPPB3 ) and just try to save it: same problem!!! I also get a red error message "Request failed with status code 503".
Edit 2
I did some more testing.
I have a website www.deijsmannetjes.nl, which is a clone of test.deijsmannetjes.nl. Also there I cannot save pages anymore once I changed them!
I have another website (zorgboerderijwillemshoeve.nl) , Joomla 5, SPPB 5, same ISP. No adaptations of memory_limit etc. There I do not have the problem.
When I export page FP-A-5 from test.deijsmannetjes.nl and import it in zorgboerderijwillemshoeve.nl, I do not have any save problems with this page!