For some reasons it could be the case that you will see invalid checker timestamps in the cmc.log and the checkmk gui:
In <2.1 we're there is no way to debug that due to the format of the state file. With Checkmk 2.1 the state file will have a different format. Anyway: If you face that issue in <2.1 you can open a case for that.
Log on to the affected site
navigate to ~/var/check_mk/core
here you will find all the "state" files
The cmc is using always the file "state"
Now rename the "state" file to state.(number)
BE AWARE: all ad hoc Downtimes + Acknowledgements + Comments are gone, but you can restore the old state file later.
The state is creating every 10 minutes or after changes in the GUI.
A backup is created when the new file is 5% smaller than the old one. Checkmk will save 30 state files and then overwrite the old ones.
The number depends on how many state.(number) files you already have.
You can either restart the whole site with
Now a new "state" file is created!
Restore a state file
Don't restart the core in that case!