Two Interlude rules that can explain an unexpected restriction.
A character cannot switch subclass if the main or subclass weight exceeds 80%.
Interlude leaves the pre-existing death XP rules unchanged at levels 1–75 and introduces a decreasing loss at higher levels. It also increases the chance and severity of the separate combat penalty for chaotic characters.
The check concerns the weight percentage on the main class or subclass, so a shared inventory can be acceptable for one class and too heavy for the other. Store unnecessary materials and try the change again. The documented rule is weight strictly above 80%; it is different from Seven Signs registration, which also checks item quantity and blocks at 80% or more.