Oh. Can I just downgrade to 2.4.0 by forcing that docker image?
Probably not. The application data was transformed during upgrade and will not be compatible with 2.4.0 anymore. You want to restore a backup.
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i had the same issue on our CEE version of 2.4.0p9 (we just did a monster upgrade from 2.0.0, which, naturally, broke things).
turns out that json decode error comes from the counter file:
/omd/sites/##site##/tmp/check_mk/counters/##hostname##
remove that file, for any offending host, and it should be good again.
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