I'm playing a bit with 2015 and I've noticed an error when opening some drawings. I've attached a screen shot of the error message.
The only work-around I've found is to open the drawing with only one sheet loaded, then to activate other sheets as I need to. After that the drawing will open normally for awhile then at some point it will load in error again.
I'm pretty sure this is 2015 related since this is occurring with files that had no problems in 2014. Also I can't recall having ever seen this error with 2014.
I got the same error on two different drawings this week. I closed solidworks (had to kill process) and re-opened the assembly then the drawing and it opened fine. I don't expect it to be the solution, but it worked for me so far.