The server has the option to require unaltered data files to prevent clients from giving themselves super powers, see g_bRequireSameAcdataMP. The files checked are:.dat files in Data\Sim\ .xml files in Data\TerrData\Objects
The server checks the hash of the clients and will not allow connection if there’s a mismatch. As it’s a hash check, even an added space will cause failure.
To help finding the culprit, a tool was added to u3. In <BMS folder>\Tools\BMS Hash Checker\ you’ll find it. The server can create a checksum file, which clients can use to find the file that is causing the issue