Let's Encrypt Invalid challenge and time out

Hi,
I have been using Let's Encrypt for a while on several VPS's. Since a few days however I'm receiving errors the certifcates caot be renewed. Both domain certificates as the server certficate. When I try to renew manual using DA, the script tells me something is wrong:

The strange thing is that when I check the Apache logs, it seems another URL is being called:
But it says status 200 and 206 (so that's ok). No further checks for the can be found in the logs.

I deleted all baed entries in BruteBlock, so it caot be Let's Encrypt servers are being blocked. I also checked out and succesfully tested the help for manually debugging https://help.directadmin.com/item.php?id=646, and this seems to be configured properly too:

I was ruing DA-version 1.51.3, until this morning, but then I updated to version 1.51.4, hoping this could solve the problem. Unfortunately it did not...

I'm ruing letsencrypt=1, /var/www/html/.well-known/acme-challenge/ exists and is set as an alias in /etc/httpd/conf/extra/httpd-alias.conf. And as the testresult already showed eveything else seems to be configured well (should be, it worked well for more than a year!). No recent maintenance was done.

Who can help me out? Thanx!
Day