The reason is that in 10.1 (
XWIKI-14758) we decided to encode upper case characters to support case insensitive systems (like Windows). Since encoded characters take more space than non encoded you can end up with a folder that cannot be migrated because it's too long for the system now.
XWiki 11 have a completely different path format which is not affected by size issues like this but the 10.1 migrated is still executed (because other migrations rely on it). Will need to refactor migration to not hit this issue during 11 migration and maybe think of what we can do for 10.11.x branch.