How did you make out with the 297MXL025G belts? I too JUST got in replacements for my Ultimaker Original and they are indeed the 300 MXL belts which are too big (by a hair). The original Ultimaker instructions were clear about making sure they were tight.
I got this response from the supplier support team:
"We do have the same thing on our UMO, that one of the belts are not as tight as the other belt (not drastically but a bit of sagging). When we brought it up to support about the issue, we were told that it was normal and could get a tensioner but didn’t need it. After 2 years, turns out they were right since the belts don’t stretch and are notched, things have been working fine even without a tensioner. "
Hoping for some direction here. Thanks!