Access your dedicated server (and be sure it is off). Open the .plr with notepad++ (right click) located here E:\Program Files (x86)\GSC2013\UserData\DedicatedServer
Find the line 92 and correct it like this Run Warmup="1"
The Practice should work by default. You can also check the .plr file for this.
Thank you to both of you. Reading about your solutions of editting the .plr file, I figured out that you can also fix it by enabling them in single player single race mode. That is probably how I disabled them in the first place. Did not expect a single player option to affect a dedicated server...