Well, as the title says...

Yup, the loop does not work, and I know for a fact that's valid opcode. It also fails on an execution module which is also more fustrating :^) thinks about life choices.

What does the tooltip say? Could it be an error on the previous page?

No, it's the first page. It says invalid opcode.
Currently trying to fix garry the robot who is saying thank you for helping me.

I have gotten the same error again:

Pretty annoying at times.

NEW INFORMATION right after:
putting a "?" symbol and then the ":" symbol back fixes it.

I'm afraid I cannot reproduce this and have no idea what it could be. I'm suspecting something about how your OS input works might potentially result in special characters? Wildly guessing here. So yeah, I'm afraid I can't help you here.