It could be a couple of things. The first thing I would say is to triple check that all the of the webassist helper files have been completely uploaded to your client's server. It sounds like they are there, but sometimes a folder gets overlooked and that would be the quick answer.
After that, I would run the PHP info command to see the php settings. My guess is that the there is a setting in the PHP ini file that is too low.
I would check the the max_execution_time and make sure it is set to something like 60. Check memory_limit. I would say 128M would be good for that... you probably won't need more. Then check post_max_size. I usually run mine around 16M. Same for upload_max_filesize. I would run that somewhere around 16M, as well. Depending on the host, I have seen some run those last two at 2M which is dismal.