To be sure, you run the Forge Server without Mods and join with a client without Mods?
If this is the case one of the Mods you are using is broken, unfortunately the logs do not contains any helpful informations.
Because of this, you probably have no choice but to use binary search to find the broken Mod.
The procedure is simple:
Remove half of the existing things, and put them aside.
Run the program / game.
Does the issue still exist?
If YES: Repeat from step 1 with the current things.
IF NO: Swap out the current things with the ones set aside, and repeat from step 1.
Repeat this process until the problematic thing/s have been found.
If you have found the Mod which cause the issue, make sure the Mod is the on with the issue by only use this Mod.
Report the Bug to the Mod author, and tell us which Mod was it (for further issues).