People are reporting a Spotify Login Error (8) since years. Spotify and DJay Pro developer can not reproduce this error. I am using DJay Pro since 1 year every day 3-5 hours without such problems also.
Some people think it has to do with multi devices logged in at same time. I have 3 devices and all are logged in at the same time. No problems.
I found this
This sound interresting. It could be acorrupt installation of Spotify. You need a clean Installation of your Operating System, drivers and Spotify. Maybe this helps.