There is a bug in url rewrite, somehow it's not being imported correctly causing some code failure. If you create any rule that is, invalid or valid and apply it, it will work again. If you delete the rule, however invalid it may be, it also works again, as the web.config structure ends up being corrected in some way.
C:\Program Files\Microsoft\Exchange Server\V15\FrontEnd\HttpProxy\owa\web.config
vDIR Export Rules
cd "C:\Windows\System32\inetsrv"
appcmd list config "Default Web Site/owa" -section:system.webServer/rewrite/rules -xml > P:\owarewriterules13-10-2021.xml
vDIR Import Rules
cd "C:\Windows\System32\inetsrv"
appcmd set config "Default Web Site/owa" -in < P:\owarewriterules13-10-2021.xml
I can say that I managed to solve my problem this way, because the failure was in the structure of the web.config imported from the backup xml by the cmd prompt itself.
But, looks like a bug.