I would suggest an opening issue for Plesk Support Team. As I wrote above, php7.3 debug build packages will be installed for the further investigation this issue directly on your server.
Thanks @IgorG
Personally, I've never had contact to Plesk support team. Where and how to do it?
Do I have to pay for it? I think, it's another type of support.
I filled the whole form and root and Plesk access, but in the end when I inserted my Plesk License Key everything was for nothing. Like it is written there in red color I'm not able to give Plesk support members access to my server, because my server is by 1und1 (now IONOS).
The first support contact since 2007 seems to be also the last one.
What makes PHP 7.3.x different than PHP 7.2.x.!?!
This might lead to an answer.
I think we all did the same: We only upgraded to 7.3.x and didn't changed our configuration.
PHP 7.3 is treating something in another way.
But I'm sure PLESK will find it out soon as possible.