This arrived today:
[Fri Jul 10 06:29:37.005301 2020] [proxy_fcgi:error] [pid 42193:tid 139976582874880] [client 126.96.36.199:57230] AH01071: Got error 'PHP message: PHP Warning: array_key_exists() expects parameter 2 to be array, string given in /var/www/html/plugins/ActivityPub/lib/models/Activitypub_notice.php on line 179
Weird... Do you have the activitypub inbox logs from the same time period?
No I haven't... and that error hasn't appeared again, so i guess it's ok to close this.
Anyway, i've configured debug log and logrotate to keep the logs more time.