Joomla URL seems to be misconfigured in Joomdle auth plugin

You have not set "joomla default language" in your joomdle config in moodle.

I did, it's en-GB (see settings in attachment)
or should I write something else here?

You should put "en" instead.

I did try a lot of languages (strings like en english, English etc etc), not one of them provided a solution; didn't help; same error occurs

I looked into the "XML-RPC Error (-32700): parse error. not well formed." error which is returned when debugging.

encountered a UTF8-encoding like solution (see link stackoverflow.com/questions/16006640/wor...rror-not-well-formed ) .. I know, its WP, but same xml-rpc method

perhaps something like this is the route to the solution...
setting language string sure is not!

the parse error is cuased by your debugging (as it breaks xml output)

If "en" does not work, there must be some problem else, but in your site it has to be set to en.