First, thanks for testing!
I think I could reproduce it. Fresh install worked but updating is a problem.
After updating CODE from 3.4 to 4.0 the updated
/etc/loolwsd/loolwsd.xml (including the new paths to collabora 6 instead of 5.3) is saved as
loolwsd.xml.rpmnew but loolwsd continues working with the old loolwsd.xml and that leads to the white screen.
Workaround until I can provide an updated module, I’ll update the wiki too.
mv /etc/loolwsd/loolwsd.xml /etc/loolwsd/loolwsd.xml.old
mv /etc/loolwsd/loolwsd.xml.rpmnew /etc/loolwsd/loolwsd.xml`
As you use a new default config file, you have to set the password again, see the wiki
I couldn’t reproduce this one, do you use a vhost for nextcloud?