I can't really tell what the problem is from the screenshot, but have you tried disabling all other triggers to see if that error still occurs?
That spawn trigger can be reduced to a few lines, if you are always spawning a fixed number of enemies. (If the number of spawned units eventually increases, you can still use this shortened spawn trigger, you would just need to create an integer variable and then replace the '15' or whatever number with the varibale, and then a simple if/then/else in your wave-end trigger would be used to determine when to increase the integer variable/number of units spawned.)
Since Round is set each at the end of each round, we could just use that to know what unit-type to spawn, without needing to check. I must have not been thinking when I suggested using all those if/then/else's.
Perhaps this would help isolate the issue, or fix whatever it was.