CMK version: 2.4.0p21
OS version: AlmaLinux 9.7
Error message:
Going to register agent at deployment server
Fetching content (using requests): https://site-fqdn/sitename/check_mk/register_agent.py
Response without json Content-Type
Unexpected answer from Checkmk server, missing json data. Please check that the credentials you entered are correct or use an automation user for the registration
See /var/lib/check_mk_agent/cmk-update-agent.log or syslog for details.
Environment:
I have a setup with 1 central site and 11 remote sites on 8 different VMs. Out of those 11 remote sites I have 2 that simply refuse to allow agents to register for updates against them.
All VMs run AlmaLinux 9.7 and Checkmk 2.4.0p21. All OS updates applied.
Monitoring works, registration for TLS communication works, access to API works for all sites.
Troubleshooting:
I have tried everything I can think of like:
- try a different username and password
- create an entirely new admin user and attempt registration
- rebuild apache configuration
- restart host
- update checkmk to latest release
I have not found anything in logs other than the following which I assume is the cause of the error from the agent but does not indicate why it’s happening on 2 of 11 remote sites:
clientIP - - [26/Feb/2026:09:08:25 +0100] “POST /SITENAME/check_mk/register_agent.py HTTP/1.1” 302 445 “-” “python-requests/2.31.0” 19359
clientIP - - [26/Feb/2026:09:08:25 +0100] “GET /SITENAME/check_mk/login.py?_origtarget=index.py%3Fstart_url%3Dregister_agent.py%253Fau_api%253D2%2526host%253Dhostname HTTP/1.1” 200 2346 “-” “python-requests/2.31.0” 27336
When the client runs the POST request against the register_agent.py URL it receives a 302 to the login page.
I can as said with the same automation user and secret run TLS registration with cmk-agent-ctl without issue. I can even run curl against the remote site API with the same username and password and get a response and login to the web page:
curl -X GET -u “automationuser:automationsecret” ``http://site-fqdn/sitename/check_mk/api/1.0/version
{“site”: “sitename”, “group”: “”, “rest_api”: {“revision”: “0”}, “versions”: {“apache”: [2, 4, 62], “checkmk”: “2.4.0p21.cee”, “python”: “3.12.11 (main, Feb 6 2026, 13:32:0
6) [GCC 14.2.0]”, “mod_wsgi”: [4, 9, 4], “wsgi”: [1, 0]}, “edition”: “cee”, “demo”: false}
I have the automationuser set with administrator permissions to see if it was a permission problem.
I have run out of ideas as to why this could be happening. On thing I tested that “kind of” bypasses the problem was to register the agent against the central site instead. That worked and was accepted (the only difference was I added -s centralsite-fqdn -i central-sitename to the registration command. It succeeds but with “unknown host”. Following this registration cmk-update-agent happily downloads the correct agent from the remote site despite being registered against the central site. Why this worked is a mystery to me.
Any ideas of what to look for to find this problem would be appreciated!