I hosted a 1.7.10 Forge server with a few mods on my PC, when my friend tried to join the server it didn't work and his game crashed, it only works for me. My friend and I have the same mods, but they are also on the server again.
Here is the error message from my friend:
The last log is:
[16:13:06] [Server thread / INFO]: Starting minecraft server version 1.7.10
[16:13:06] [Server console handler / ERROR]: Exception handling console input
java.io.IOException: The handle is invalid
at java.io.FileInputStream.readBytes (Native Method) ~ [?: 1.8.0_171]
at java.io.FileInputStream.read (Unknown Source) ~ [?: 1.8.0_171]
at java.io.BufferedInputStream.read1 (Unknown Source) ~ [?: 1.8.0_171]
at java.io.BufferedInputStream.read (Unknown Source) ~ [?: 1.8.0_171]
at sun.nio.cs.StreamDecoder.readBytes (Unknown Source) ~ [?: 1.8.0_171]
at sun.nio.cs.StreamDecoder.implRead (Unknown Source) ~ [?: 1.8.0_171]
at sun.nio.cs.StreamDecoder.read (Unknown Source) ~ [?: 1.8.0_171]
at java.io.InputStreamReader.read (Unknown Source) ~ [?: 1.8.0_171]
at java.io.BufferedReader.fill (Unknown Source) ~ [?: 1.8.0_171]
at java.io.BufferedReader.readLine (Unknown Source) ~ [?: 1.8.0_171]
at java.io.BufferedReader.readLine (Unknown Source) ~ [?: 1.8.0_171]
at net.minecraft.server.dedicated.DedicatedServer $ 2.run (DedicatedServer.java:97) [lv.class:?]
[16:13:10] [Server thread / INFO]: Loading properties
[16:13:10] [Server thread / INFO]: Default game type: SURVIVAL
[16:13:10] [Server thread / INFO]: Generating keypair
[16:13:10] [Server thread / INFO]: Starting Minecraft server on *: 25565
[16:13:13] [Server thread / INFO]: Preparing level "world"
[16:13:13] [Server thread / INFO]: Preparing start region for level 0
[16:13:14] [Server thread / INFO]: Done (1,475s)! For help, type "help" or "?"
[16:18:57] [User Authenticator # 1 / INFO]: UUID of player NussTastatur557 is b0b49e0f-f402-4870-a74b-dbc7369660ff
[16:18:58] [Server thread / INFO]: NussTastatur557 [/192.168.178.32:58033] logged in with entity id 70 at (-54.18207470376421, 45.0, 98.05369301391511)
[16:18:58] [Server thread / INFO]: NussTastatur557 joined the game
[16:31:09] [Server thread / INFO]: NussTastatur557 lost connection: TextComponent {text = 'Disconnected', siblings = [], style = Style {hasParent = false, color = null, bold = null, italic = null, underlined = null, obfuscated = null, clickEvent = null, hoverEvent = null}}
[16:31:09] [Server thread / INFO]: NussTastatur557 left the game
[16:33:58] [Server thread / INFO]: Stopping server
[16:33:58] [Server thread / INFO]: Saving players
[16:33:58] [Server thread / INFO]: Saving worlds
[16:33:58] [Server thread / INFO]: Saving chunks for level 'world' / Overworld
[16:33:58] [Server thread / INFO]: Saving chunks for level 'world' / Nether
[16:33:58] [Server thread / INFO]: Saving chunks for level 'world' / The End
Based on the error message, I would say that a mod in the server and with you is different than with your friends