These are the log-entries for the error. Last Time i think it was a redirection issue with a weg.config entry caused by AdminTools.
GET /moodle/auth/joomdle/land.php - 80 - 216.244.66.237 Mozilla/5.0+(compatible;+DotBot/1.2;++https://opensiteexplorer.org/dotbot;+help@moz.com) - 500 50 5 161
GET /moodle/auth/joomdle/land.php username=lisalampl%40gmx.at&token=dd32c9cdb3600e49e7d9b644095aa1fe&use_wrapper=0&create_user=0&wantsurl=%2F 443 - 104.28.130.6 Mozilla/5.0+(iPhone;+CPU+iPhone+OS+15_5+like+Mac+OS+X)+AppleWebKit/605.1.15+(KHTML,+like+Gecko)+Version/15.5+Mobile/15E148+Safari/604.1 - 500 50 5 15
GET /moodle/auth/joomdle/land.php username=lisalampl%40gmx.at&token=dd32c9cdb3600e49e7d9b644095aa1fe&use_wrapper=0&create_user=0&wantsurl=%2F 443 - 104.28.130.6 Mozilla/5.0+(iPhone;+CPU+iPhone+OS+15_5+like+Mac+OS+X)+AppleWebKit/605.1.15+(KHTML,+like+Gecko)+Version/15.5+Mobile/15E148+Safari/604.1 - 500 50 5 0
GET /moodle/auth/joomdle/land_logout.php - 443 - 10.10.10.213 - - 500 50 5 0
GET /moodle/auth/joomdle/land.php username=vzlatanova&token=c36986c08239f57891ade76fdd504215&use_wrapper=0&create_user=0&wantsurl=%2Ffreunde%2Fanmeldung%3Ftask%3Duser.login 443 - 84.115.234.15 Mozilla/5.0+(Macintosh;+Intel+Mac+OS+X+10_15_7)+AppleWebKit/537.36+(KHTML,+like+Gecko)+Chrome/103.0.0.0+Safari/537.36 www.kepos.at/freunde/anmeldung 500 50 5 0
GET /moodle/auth/joomdle/land.php username=vzlatanova&token=8b3746ed2c8ef31a8ff526058ed8754e&use_wrapper=0&create_user=0&wantsurl=%2Ffreunde%2Fanmeldung%3Ftask%3Duser.login 443 - 84.115.234.15 Mozilla/5.0+(Macintosh;+Intel+Mac+OS+X+10.15;+rv:91.0)+Gecko/20100101+Firefox/91.0 kepos.at/ 500 50 5 15

2022-08-05 05:41:12 10.10.10.213 POST /freunde/anmeldung task=user.logout 443 - 84.115.234.15 Mozilla/5.0+(Macintosh;+Intel+Mac+OS+X+10.15;+rv:91.0)+Gecko/20100101+Firefox/91.0 kepos.at/freunde/anmeldung 303 0 0 171
2022-08-05 05:41:12 10.10.10.213 GET /moodle/auth/joomdle/land_logout.php - 443 - 84.115.234.15 Mozilla/5.0+(Macintosh;+Intel+Mac+OS+X+10.15;+rv:91.0)+Gecko/20100101+Firefox/91.0 kepos.at/ 500 50 5 15

That is the access.log, but it does not show any useful info. You should look at the error.log.

Its a error from w3svc on windwos server. Its a 500.50 (ReWrite) error. I enabled tracking and got this:
Detaillierte Fehlerinformationen:
Modul RewriteModule
Benachrichtigung BeginRequest
Handler php-8.0.20
Fehlercode 0x80070005
Angeforderte URL www.kepos.at:443/moodle/auth/joomdle/lan...%3Ftask%3Duser.login
Physischer Pfad C:\inetpub\websintern\kepos\moodle\auth\joomdle\land.php

and in the logs:
0 1 0 1 0x0 MARS {80000EB4-0000-CB00-B63F-84710C7967BB} 5 kepos 1610616498 0 www.kepos.at:443/moodle/auth/joomdle/lan...%3Ftask%3Duser.login GET GENERAL_REQUEST_START {D42CF7EF-DE92-473E-8B6C-621EA663113A} 0 1 5 53 0x0 MARS {80000EB4-0000-CB00-B63F-84710C7967BB} AspFilterSessionId true GENERAL_SET_REQUEST_HEADER {D42CF7EF-DE92-473E-8B6C-621EA663113A} 0 1 4 55 0x0 MARS {80000EB4-0000-CB00-B63F-84710C7967BB} 192.168.69.29 50216 10.10.10.213 443 GENERAL_ENDPOINT_INFORMATION {D42CF7EF-DE92-473E-8B6C-621EA663113A} 0 1 4 50 0x0 MARS {80000EB4-0000-CB00-B63F-84710C7967BB} Connection: keep-alive Accept: text/html,application/xhtml+xml,application/xml;q=0.9,image/avif,image/webp,*/*;q=0.8 Accept-Encoding: gzip, deflate, br Accept-Language: de,en-US;q=0.7,en;q=0.3 Cookie: MoodleSession=8k1vsbrtbhtluplr80o3gnkcpd; _ga=GA1.2.583402735.1659694588; wc_visitor=63997-d70a32b3-8e9d-a00a-25ce-4e82b91e46a4; wc_client=kepos.at+..+referral+..++..++..++..++..+https%3A%2F%2Fwww.kepos.at%2Ffreunde%2Fanmeldung+..+63997-d70a32b3-8e9d-a00a-25ce-4e82b91e46a4+..+; 8a9a3e7f0a0993110cd0277bf8f5cce0=nahmrcbhcuqfl4a8n8l0r7sroe; 9586433672ff46d12fbfdd126a171c11=p3vlah1noiv38bkkkkuhkekubn; wc_client_current=direct+..+none+..++..++..++..++..+https%3A%2F%2Fwww.kepos.at%2F+..+63997-d70a32b3-8e9d-a00a-25ce-4e82b91e46a4+..+; _gid=GA1.2.1055581242.1659953928; joomla_user_state=logged_in; jch_optimize_no_cache_user_state=user_logged_in Host: www.kepos.at Referer: www.kepos.at/freunde/anmeldung User-Agent: Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:103.0) Gecko/20100101 Firefox/103.0 Upgrade-Insecure-Requests: 1 Sec-Fetch-Dest: document Sec-Fetch-Mode: navigate Sec-Fetch-Site: same-origin Sec-Fetch-User: ?1 GENERAL_REQUEST_HEADERS {D42CF7EF-DE92-473E-8B6C-621EA663113A} 0 1 4 30 0x0 MARS {80000EB4-0000-CB00-B63F-84710C7967BB} 513 GENERAL_GET_URL_METADATA {D42CF7EF-DE92-473E-8B6C-621EA663113A} 0 1 4 43 0x0 MARS {80000EB4-0000-CB00-B63F-84710C7967BB} php-8.0.20 FastCgiModule C:\PHP-8.0.20\php-cgi.exe HANDLER_CHANGED {D42CF7EF-DE92-473E-8B6C-621EA663113A} 0 1 4 56 0x0 MARS {80000EB4-0000-CB00-B63F-84710C7967BB} X-Powered-By ASP.NET false GENERAL_SET_RESPONSE_HEADER {D42CF7EF-DE92-473E-8B6C-621EA663113A} 0 1 4 56 0x0 MARS {80000EB4-0000-CB00-B63F-84710C7967BB} X-Content-Type-Options nosniff false GENERAL_SET_RESPONSE_HEADER {D42CF7EF-DE92-473E-8B6C-621EA663113A} 0 1 0 33 0x0 MARS {80000EB4-0000-CB00-B63F-84710C7967BB} 500 50 500.htm GENERAL_SEND_CUSTOM_ERROR {D42CF7EF-DE92-473E-8B6C-621EA663113A} 0 1 4 56 0x0 MARS {80000EB4-0000-CB00-B63F-84710C7967BB} Content-Type text/html true GENERAL_SET_RESPONSE_HEADER {D42CF7EF-DE92-473E-8B6C-621EA663113A} 0 1 4 35 0x0 MARS {80000EB4-0000-CB00-B63F-84710C7967BB} GENERAL_FLUSH_RESPONSE_START {D42CF7EF-DE92-473E-8B6C-621EA663113A} 0 1 4 47 0x0 MARS {80000EB4-0000-CB00-B63F-84710C7967BB} Content-Type: text/html Server: Microsoft-IIS/8.5 X-Powered-By: ASP.NET X-Content-Type-Options: nosniff GENERAL_RESPONSE_HEADERS {D42CF7EF-DE92-473E-8B6C-621EA663113A} 0 1 5 49 0x0 MARS {80000EB4-0000-CB00-B63F-84710C7967BB} www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd "> www.w3.org/1999/xhtml "> GENERAL_RESPONSE_ENTITY_BUFFER {D42CF7EF-DE92-473E-8B6C-621EA663113A} 0 1 4 36 0x0 MARS {80000EB4-0000-CB00-B63F-84710C7967BB} 1401 0 GENERAL_FLUSH_RESPONSE_END Der Vorgang wurde erfolgreich beendet. (0x0) {D42CF7EF-DE92-473E-8B6C-621EA663113A} 0 1 0 2 0x0 MARS {80000EB4-0000-CB00-B63F-84710C7967BB} 1401 1324 500 50 GENERAL_REQUEST_END {D42CF7EF-DE92-473E-8B6C-621EA663113A}

Ok foound it. I remembered that we had a similar problem when i checkes the status code (Acess-denied). Problem was that there is a strange security isse on the joomdle folder. The Owner of the folder is IUSR instead of a local Administrator Account. IUSR gets the full access to webroot directory which is very strange because in that case gets MORE rights then usual but produces an error in the Rewrite attempt. We changed to Owner of the joomdle directory to local Admin and now it works.

Great, thanks for letting us know about the solution.