It is possible to store backpacks in every backpack except in the combined backpack, which you can't put anything in.

the version of the plugin is BackpacksRemastered-2020.0.5, and the server is an Aternos server using Spigot 1.16.5

You store backpacks in the combined backpack by combing it in a crafting grid. It does not store items directly. This is intended.