Create a file on UserData/Scripts/Modes/Trackmania/collisions.Script.txt
, with the content on file linked with this gist
Then you need to edit your matchsettings file like this :
<playlist>
<gameinfos>
<script_name>Trackmania/collisions.Script.txt</script_name>
</gameinfos>
<map>
<file>[YOUR MAP FILE HERE]</file>
</map>
</playlist>
Then load it.
For information: collisions on servers are not recommended on public servers (very buggy...) I advise you to use it on private servers with your friends (in RPG/Trial session for example)
Additional: if all players have a good connection, you can set these values to your dedicated config to reduce weird collisions :
<packetassembly_packetsperframe>0</packetassembly_packetsperframe>
<packetassembly_fullpacketsperframe>0</packetassembly_fullpacketsperframe>
<delayedvisuals_s2c_sendingrate>255</delayedvisuals_s2c_sendingrate>
<trustclientsimu_c2s_sendingrate>255</trustclientsimu_c2s_sendingrate>