After resetting Confluence import on Test Installgetting errors at Step 4

Hello,
with tests on Confluence import ongoing in a 14.10.11 install, I had to reset the import a pair of times, and now I get this error at Step 4 after importing anew the Confluence export zip archive.
I remind the context:
A Debian 11 install on a remote Virtual Machine with XWiki 14.10.11, tomcat9 and PostgreSQL.

Previously, on fresh install the import had not thrown errors after importing and I could reach the Step 4 (before meeting with other annoyances).

(it is a long log, can you get anything useful out of it?):
log.txt (121.3 KB)

Also once at Step 4 : what does the “Compute” button feature do exactly?

This is a real question because from the beginning of the Confluence import tests I have been doing, it is when I get stuck each time and therefore I can’t finish the import test (reminding what I import, it is an about 500MB zip file, just one Space exported from Confluence, whereas ultimately we will have to import several dozens of them).

Thanks for your help, best regards,
Joyce MARKOLL