Jscript Addin:Windows Client: OK, but Webclient: "Method XYZ was not found in the control add-in"

Dear colleagues,

I've created a simple javascript add-in for NAV2016 that works fine in the Windows client.
It's just a textarea with an event and custom methods to set and get the textarea content.

The error message "method "my-method-name" was not found in the control add-in" comes up when using the same page in the web client.
I've double checked allmost everything+
-restarted the NST and IIS sebsite several time
- deleted all add-ins
- deleted the object from the object metadata table
- ...
- if I change the action to another method, the message changes, too: method "the-other-method name" was not found...
-
The error is not very popular in google, so I hope that there is a expert in the community, who had this error in the past and could solve it.

Thanks in advance
Sebastian