I have tried to do everything (change the password, remove the password, recover password on mydomoticz, remove api key on local domoticz) - still the same error in log "PROXY: Authenticate result: wrong password for apikey/instanceid combination.":

My Instance ID on local domoticz is NOT empty and ...

I can't see Instance ID in MyDomoticz because by local domoticz is not registered in MyDomoticz (because of password / API key).
In MyDomoticz I have only option to see my API key and password / name change.

In MyDomoticz Dashboard instead of User icon I see only "Enter your api data to your own ...

I Have the same error in the log: PROXY: Authenticate result: wrong password for apikey/instanceid combination.
I'm sure that password and API key are correct.
In my opinion mydomoticz has a problem with unique Instance ID. When I have tried to change Enabled Shared, Domoticz Allowed, Apps Allowed ...