That looks like the error Tasker triggers when the input variable in a search/replace action doesn't exist. You can try that yourself by creating a task that does your search/replace, but instead of %evtprm3, put in %nosuchvar. The error message isn't as clear as it could be, but you'll see that it occurs when the input variable isn't defined.
So, how sure are you that %evtprm3 is populated at the beginning of the task? Try adding a Alert/Popup step to show the value of %evtprm3 as the first step in your task.