If interacting with villagers in a village updates the Players NBT tag in villages.dat, two subtags are created: Int S and String UUID. S is given an integer value, and UUID is populated with the player's UUID.

If these subtags are loaded from the villages.dat file during gameplay, the game spams the following error to the console when attempting to save data to villages.dat:

java.lang.IllegalArgumentException: Name and ID cannot both be blank
	at com.mojang.authlib.GameProfile.<init>(GameProfile.java:25)
	at ml.a(SourceFile:81)
	at ml.a(SourceFile:140)
	at vv.b(SourceFile:455)
	at vx.b(SourceFile:228)
	at baj.a(SourceFile:92)
	at baj.a(SourceFile:78)
	at ls.a(SourceFile:927)
	at ls.a(SourceFile:879)
	at net.minecraft.server.MinecraftServer.a(SourceFile:372)
	at bzl.a(SourceFile:214)
	at bzl.C(SourceFile:145)
	at net.minecraft.server.MinecraftServer.run(SourceFile:458)
	at java.lang.Thread.run(Thread.java:745)

Deleting either the entire villages.dat file or the contents of the Players NBT tag from each villager corrects this problem, until player interaction populates the tag again.

This may be somehow related to the fix for MC-101325 Loading Village.dat containing player reputation causes NullPointerException