To be clear, I don’t immediately think you are in breach of any policy. You’re using some map tiles (with accreditation) with no SLA so if you get blocked from using them in the future (unlikely, but possible) you won’t be able to complain.

To solve the other issue you’ll need to use the usual web browser debug tools. In Firefox it’s menu / web developer / browser console (other browsers will differ). You’ll see any errors in there - and it’s quite possible that they’re not related to the OSM tiles at all but to the other stuff that that site is doing.