Two data points from this week. Both as EuroBonus Gold members on SAS Go Light Fare within Europe:

Website wanted to charge €35 when trying to add a bag.

HAM-CPH-LLA: Agent told that system would be wrong but she'd know that one bag is free as EBG. Looks like she overwrote it in the system somehow and was able to check it for free.

LLA-ARN-BER: Agent asked whether we'd have any bag to check. Said one, agent checked it for free, without any issue.

In both cases, the bag was carry-on sized. Not sure, if that may make any difference. Looking forward to any reports from self check-in or CPH/ARN/OSL agents.

Edit: The app is now stating "no carry-on" for the passenger that checked a bag. So it appears that there's indeed an option to check the carry-on for free?