Looks like you’re filesystem’s corrupted. And in that case Linux remounts it in read-only mode. Rebooting your PI might work, as it will trigger a filesystem check on boot. If that doesn’t fix it, you can try to put your memory card in a PC with Linux and see if it’s possible to fix the FS from there. If not, you may be able to recover your Check_MK installation from it and start with a new OS installation on your pi.
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