I am using WF5/Creo. The number of files/drawings to be opened at one time is 15. Is there a reason that you can't have more than this?
Also, when editing a weld symbol - it treats the window as a 16th window. I recieved an error message that I had to many windows open. Try closing a few. I had 15 windows of files open and when I closed one, I was able to edit the weld symbol.
Make the weld symbol editing box not the same type of feature/structure so that you if you have the drawing open currently, you would not get an error and the window would open normally for editing.
April 11, 2013: Beside the window for welding, I just found that if I wanted to add a symbol, that window counts for one of you 15 windows too. I wasn't paying attention to the text line and clicked on the symbol icon, and it wasn't doing anything but silently screaming at me that I have to many windows already open.
May 8, 2014: Adding an annotation that require a pop-up window is considered the 16th window also. Stop what you're doing, figure out what window you can close out, go back to adding the annotation that you need