I have a setsing like below: On the main side is a special testing area behind an own router (RB1100AHx2) that is connected with the main router (CCR1009-8G-1S-1S) on the side over a production network.
There are 3 other sides connected via VPN and the routes are distributed over the VPN with OSPF.
Now the 3 sides sholud get access to the testing area. The firewalls are set up correspondingly.
My question is how I can distribute the networks from the testing area over OSPF when the connection between the main router and the testing router is part of the regular area of the main site and not a backbone area?

OSPF virtual link, or L2 tunnel that connects edge router to the backbone.

Just note that 'all areas must connect to the backbone' still occurs.... Even with virtual links. So if you have say Area2->Area1->Area0 then guess how Area2 and Area1 communicate? Nope you guessed wrong

Area2 will tunnel traffic to Area0 then back to Area1......
....yes OSPF design sucks and yes we need IS-IS or EIGRP. Nudge nudge wink wink Mikrotik.... No seriously ffs why do we still not have ISIS because the majority of your customer base would benefit from the far more logical design and traffic flow

The area ID of the production network is 0.0.0.2 and both router find another and exchange routes.
The configuration of the virtual link is not complicated, set the Neigbor ID of the other router and and chose the production area for transit (first renounced encryption).
But I get an error on the test router " Discarding packet, mismatched area ID" "mine=0.0.0.2" "recived=0.0.0.0""source=<IP-Main router>".
I have exported the configuration for the area from the main router and imported in to the test router.
On the test router isn't a area with the ID 0.0.0.0 but it is the ID of the backbone area on the main router.

Is this the effect that you mean millenium7?
I thought the virtual link wiould allow me to connect areas over a other area to the backbone.

For the sake of completeness, I would like to add that both routers use Version 6.49.