After Server Update from 3.3.1 to 3.3.3 the Server log is spammed with:
[Server thread/ERROR] [minecraft/ChunkSerializer]: Unknown structure start: valhelsia_structures:small_dungeon

Minecraft Version: 1.16.5
Forge Version: 36.1.25
Valhelsia Version: 3.3.3
V. Structures Version: 0.1.2

A log filter had been implemented in release 0.1.3.

I have the same thing. Spam like crazy.

Minecraft 1.16.5
Forge 36.1.15
Vahelsia Structures 0.1.2

Same issue here. Server was updated from 3.3.2a to 3.3.3.

Minecraft version 1.16.5
Forge version 36.1.25
Valhelsia version 3.3.3
Valhelsia Structures version 0.1.2

Same issue.

Minecraft 1.16.5
Forge 36.1.0
Vahelsia Structures 0.1.2

Anybody updating from an old world (from before Valhelsia Structures 0.1.1) to a new one will have this issue, unfortunately. It's a side effect of how Forge deals with removing structures from existing worlds (the small dungeon was removed + replaced with a different dungeon). We can most likely add a workaround for the next release though to avoid this, since it's easy enough to just add a "dummy" dungeon to prevent the log spam.

It's actually pretty easy to add log suppression for any structure or structure piece that you removed. First make a class with the same annotation, extends AbstractFilter, and implements the LogEvent method. In there, do matching for your mod's mod id for the "Unknown structure piece" and ""Unknown structure start" messages and do return Result.DENY; if so. https://github.com/TelepathicGrunt/RepurposedStructures/blob/3075bcb64aed4819d3f0c800c05bdfceb29ff057/src/main/java/com/telepathicgrunt/repurposedstructures/utils/LogSpamFiltering.java#L62

Then add the class to the logger in mod init
https://github.com/TelepathicGrunt/RepurposedStructures/blob/3075bcb64aed4819d3f0c800c05bdfceb29ff057/src/main/java/com/telepathicgrunt/repurposedstructures/RepurposedStructures.java#L138

Hope this helps! It is a better solution than doing dummy structures or pieces so that they don't show up still in /locate and Forge already makes the world safe for structure remove. Just the logspam needs to be suppressed.