Menu "Connections" is missing and calls in custom functions to other Zoho services fail with code 14

Menu "Connections" is missing and calls in custom functions to other Zoho services fail with code 14

Greetings,

since three days the REST calls to Zoho Books via custom functions fail with the following error message: 
{"code":14,"message":"Invalid value passed for authtoken."}

I wanted to refresh or renew the integration with Zoho Books but noticed the menu "Connections" missing under "Developer Space". I am not quite sure what to do now.