Minecraft: Can't use mod blocks in my multiplayer server

Actually you have to double check if server has SAME versions of mods and if both client and server has SAME mods AND configs.

If client misses some blocks the server has, it won't let you in, however if your client has more mods than server, everything is fine, the client will show you items in crafting etc, however if you really want to get it - server does not know what you do and kicks you out, preventing corruption of data or crash.

So when you say you made some new things by that mod, check that those things got added by server, too.

If everything seems to be good, but crashes are still there - then you have to check the client&server logs&crashlogs. As I know, this page should not be technical help, however be free to contact me with them if you need.