A player joined with versions of the server's mods that the server refuses

Your log will have other names, versions and times. Search a line of it from all failures.

What it means

Mods that add things to the game talk to the player’s game over their own network channels, and each channel carries a protocol version. When a player joins a Forge server, the server compares the versions of the player’s channels with its own. If a mod’s version on the player’s side is one the server does not accept, Forge lists those channels at ERROR and ends the join; the player sees a disconnect screen about mismatched mods.

The server itself is fine and keeps running for everyone else. This is not the line rejected vanilla connections, which Forge prints when a game without Forge (or a scanner on the internet) connects; the diagnosis does not report that one.

Why it happens

The server’s mods were updated and the player’s were not, or the other way round.
The player installed the mods by hand and picked other versions than the server’s.
The player uses another modpack or an older copy of the server’s pack.

How to fix

Read channels: the part before each colon is usually the mod’s id, such as farmersdelight. Those are the mods whose versions differ.
Give the player the exact versions the server runs: the same modpack version, or the same jar files from the server’s mods folder.
If many players hit it after an update, tell them to update the modpack in their launcher before joining.
The player’s own client-only mods (minimaps, shaders) are not the cause; only mods the server also has are compared.

In Perland

The issue appears in the server’s Diagnosis tab with the channels Forge refused. On a server whose mods come from a modpack, the server’s invite page and the player pack link offer players the pack the server runs, to import into their launcher, so their mods match.

References

Forge documentation: SimpleImpl and protocol versions

Details the diagnosis reports

Perland's server diagnosis returns this failure as the code client_mods_mismatch with these parameters, each present only when the log names it.

Parameter	Meaning
`channels`	The network channels Forge refused, as it lists them (each `<mod id>:<name>`, separated by commas); the part before the colon is usually the mod's id.

Log lines this page covers

Channels [<channels>] rejected their client side version number