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To use USB for communications purposes you need to access system services reserved and accessible by root only. (Trying to arbitrate between users vying for the same physical I/O port would to be extremely messy and only one user could win.
So... root only.)
(If I've got that wrong someone will correct me no doubt, but any time I’ve run Minicom on my netbook via a USB to RS232 serial converter, I've needed root access via sudo.)
The change in your graphics is interesting as it shouldn't happen at all!
You're probably going to have to share some of your code so we can see exactly what's going on.