The bug

Saving a structure with a name containing two consecutive dots (..) logs the error

Couldn't load structure <structure_name>: java.io.IOException: Invalid relative path to resource: <resource_path>

This is likely caused by the code trying to load the structure from data packs. Loading them from and saving them to the generated/<namespace>/structures folder of a world works fine.

Either way two consecutive dots can form perfectly valid paths when they are in the middle (e.g. a..b) or at the beginning (e.g. ..a) of a path piece, therefore this check should be more precisely and the error message should include, that this is data pack related only.

How to reproduce

1. /setblock ~ ~ ~ structure_block[mode=save]{mode:"SAVE",name:"a..b"}
   

2. Open the structure block GUI and click "SAVE"
3. Look at the log
   → The following error appeared

   Couldn't load structure minecraft:a..b: java.io.IOException: Invalid relative path to resource: minecraft:structures/a..b.nbt