Menu Connections missing and calls to other Zoho Services via custom functions fail with code 14

Menu Connections missing and calls to other Zoho Services via custom functions fail with code 14

Greetings,

since three days our custom invoice creation functions fail when trying to execute REST calls to Zoho Books. The error message is as follows:
{"code":14,"message":"Invalid value passed for authtoken."}

I wanted to refresh or redo the integration to Zoho Books but noticed the "Connections" menu missing entirely from Developer Space. I am a bit lost on what to do...