Hello ErnestoM,
Thank you for your reply. I did like you said, forced it to open in InfoPath. And now I have more detailed error here. Frankly, I don't quite understand what permission it requires. I'm publishing the form using sysadmin account, and I'm trying to open it using sysadmin account too. This account has most full rights/permissions.
The form cannot be opened because it requires the domain permission level and it currently has restricted permission. To fix this problem, open the form from the location it was published to. Forms that require the domain permission level contain features that access information on a network, such as data connections, linked images, and code.
It is strange because I made a single change, which is I used dynamic XML Data Connection (http://portal/_vti_bin/owssvr.dll?Cmd=Display&List={ListGuid}&View={ViewGuid}&XMLDATA=TRUE) instead of static one (simple SharePoint list Data Connection) that I had before. They are both pulling from the same list.
Well, I think there should be an easy solution, as the form opens in Preview without problem, but I feel like I cannot handle this one without your help. Please help me if you know what else I could do to try to resolve it.
Thank you!