@huvumus, some time ago I looked into OptimizePress and I seem to remember they said it was not compatible with Multisite. However, after looking into again they say it is compatible but with some steep caveats. In other words, they certainly aren’t making it easy.
I really doubt the problem is with the Domain Mapping plugin to be honest.
You can always test the problem by turning off Domain Mapping and see if the problem goes away. If you don’t own the multisite yourself then you should get your money back from them. Read what they say here:
https://optimizepress.zendesk.com/hc/en-us/articles/200329666-Does-OptimizePress-support-Wordpress-Multisite