This is indeed a serious ML compatibility bug.
Line 42 of ModLoaderConnectionHandler is:
if (!ModLoaderHelper.sidedHelper.clientConnectionClosed(manager, mod))
This is within method connectionClosed(INetworkManager), who's javadocs clearly states it will be called on BOTH CLIENT AND SERVER (IConnectionHandler.java:56)
On the client side, ModLoaderHelper.sidedHelper is set to an instance of ModLoaderClientHelper upon said instance's construction. However, on the server side, ModLoaderHelper.sidedHelper is never set and defaults to null.
It may be notable that all usages of ModLoaderHelper.sidedHelper within the class ModLoaderHelper itself is guarded by null checks (ModLoaderHelper.java:127, ModLoaderHelper.java:159).
Hope this helps!