@Philipp: even if the gpu-manager started on boot, it wouldn't solve your problem. Here's why (as your log shows):
"Proprietary driver installer detected"
I assume you installed fglrx from the AMD installer without generating the deb packages. By doing so, you probably broke your system.
Also, I don't know if they blacklist the gpu-manager in Linux Mint 17 (I don't use Mint).
@Philipp: even if the gpu-manager started on boot, it wouldn't solve your problem. Here's why (as your log shows):
"Proprietary driver installer detected"
I assume you installed fglrx from the AMD installer without generating the deb packages. By doing so, you probably broke your system.
Also, I don't know if they blacklist the gpu-manager in Linux Mint 17 (I don't use Mint).