Me as host:

16:40:07.970 INFO spireTogether.SpireTogetherMod> Client: Player: URANS1K; Version: 6.4.13; UniqueID: 2
16:40:08.051 INFO helpers.ImageMaster> [WARNING] No image at spireTogetherResources/images/charSkins/Silent/red/shoulder.png
16:40:08.051 INFO helpers.ImageMaster> [WARNING] No image at spireTogetherResources/images/charSkins/Silent/red/shoulder2.png
16:40:08.055 INFO helpers.ImageMaster> [WARNING] No image at spireTogetherResources/images/charSkins/Silent/red/corpse.png
16:40:15.597 INFO helpers.ImageMaster> [WARNING] No image at spireTogetherResources/images/charSkins/Silent/red/shoulder.png
16:40:15.597 INFO helpers.ImageMaster> [WARNING] No image at spireTogetherResources/images/charSkins/Silent/red/shoulder2.png
16:40:15.599 INFO helpers.ImageMaster> [WARNING] No image at spireTogetherResources/images/charSkins/Silent/red/corpse.png
16:40:25.470 INFO core.CardCrawlGame> RESUME()
16:40:29.113 INFO spireTogether.SpireTogetherMod> NON_FATAL_CONNECTION_BROKEN: PFServerClient 2 socket connection broken due to heartbeat timeout
16:40:29.113 INFO spireTogether.SpireTogetherMod> Server: Disconnecting client of ID 2 under index 1
16:40:29.115 INFO spireTogether.SpireTogetherMod> Terminated PFServerClient of ID 2
16:40:29.115 INFO spireTogether.SpireTogetherMod> PFServerClient 2 listener terminated.
16:40:51.569 INFO core.CardCrawlGame> PAUSE()

Friend:

Mods in stacktrace:

anniv6 (1.4.0)
Cause:
java.lang.IllegalArgumentException: n must be positive
at com.badlogic.gdx.math.RandomXS128.nextLong(RandomXS128.java:110)
at com.badlogic.gdx.math.RandomXS128.nextInt(RandomXS128.java:99)
at com.megacrit.cardcrawl.random.Random.random(Random.java:71)
at spireMapOverhaul.zones.candyland.CandyLand.modifyRewardCards(CandyLand.java:66)
at spireMapOverhaul.patches.interfacePatches.RewardModifierPatches$ModifyRewardsPatch.ModifyRewards(RewardModifierPatches.java:68)
at com.megacrit.cardcrawl.rooms.AbstractRoom.update(AbstractRoom.java:453)
at com.megacrit.cardcrawl.dungeons.AbstractDungeon.update(AbstractDungeon.java:2532)
at com.megacrit.cardcrawl.core.CardCrawlGame.update(CardCrawlGame.java:876)
at com.megacrit.cardcrawl.core.CardCrawlGame.render(CardCrawlGame.java:423)
at com.badlogic.gdx.backends.lwjgl.LwjglApplication.mainLoop(LwjglApplication.java:225)
at com.badlogic.gdx.backends.lwjgl.LwjglApplication$1.run(LwjglApplication.java:126)

We at 2 act and we play with all mods like before so it's not our few runs.
If my friend trying to connect for long time, he was just kicked.
I'm just seeing his character and then in few seconds he says "BYE!"
Maybe you can add something like config ping system or something (at least we think that a reason)