I have seen this error many times on iOS 11 and OO v3.3.1. See here: https://discourse-test.omnigroup.com/t/paste-now-broken-in-ios-11-3-x-a-ios-bug-workaround-fix-in-oo-v3-1-1-now-available-and-ios-12-beta/38116?
I have not yet seen it on iOS 12 and was curious if it has been solved. The problem is the paste function. You can paste in other apps without problems, but not into OO. Killing OO and restarting it solves the problem (temporarily). Therefore I suspect a memory problem but Omni has not yet been able to track it down.
Once I also had the problem (on iOS 11 & 12), that OO didn’t open an oo-file I had stored on my iCloud Drive. It loaded the file and went straight back to the open dialogue. Here killing and reopening the app also fixed the problem.
You’re right, which is kind of weird since while I don’t typically visit random links I generally check to see where they lead to. And these are pretty blatant. I wonder if they didn’t load visually as I’ve been suffering through some broken internet lately and pages often time out when partially loaded. Oh well, no harm done.