[10:17:18] [Netty Server IO #3/INFO] [FML]: Attempting connection with missing mods [mercurius_updater] at CLIENT
[10:17:18] [Netty Server IO #3/INFO] [journeymap]: CLIENT
[10:17:58] [Netty Server IO #3/ERROR] [FML]: NetworkDispatcher exception
io.netty.handler.timeout.ReadTimeoutException: null
[10:17:58] [Server thread/INFO]: Patrick_The_Star lost connection: Timed out
[10:17:59] [Server thread/INFO]: Patrick_The_Star left the game

I had this, a lot, with previous versions. As stupid as it may sound, several windows reboots and switching between server connect and direct connect helped somewhat. It's not a mod pack problem, but a long known bug in minecraft: https://bugs.mojang.com/browse/MC-34947

I did not have the problem with the latest modpack version in the last few days.

anything timeout related might be helped by adding the java JVM start args to both the server AND your client to increase timeout (default is 30 seconds):

-Dfml.readTimeout=90