This is caused by a bug when saving the translation file concerning the langauge key for “Let’s Encrypt log”.
When the translation file for the Monitor tab is saved, the necessary backslash before the ‘ in “Let’s” is not saved into the file. The problem is described in this bug report: https://git.ispconfig.org/ispconfig/ispconfig3/-/issues/5881 but for the moment not resolved.
To workaround this problem, manually edit the file /usr/local/ispconfig/interface/web/monitor/lib/lang/XX.lng (where XX is your language). Find the line beginning with:
$wb['Let's Encrypt log'] =
Insert a backslash like this:
$wb['Let\'s Encrypt log'] =
Save the file and your Monitor tab will work again. This must be repeated each time you edit the translations for monitor.