I am receiving a similar error. I have LoadRunner 12.50 with Patch 3.
After I updated to Patch 3, and when I create a new HTTP/HTML Web script, I am receiving the "An error has occured" pop during the replay of any new script that I create.
I launched Vugen as administrator and still receive the same issue.
However, for existing scripts that I generated prior to the patch 3 update, I do not receive the error during replay.