Thanks for reaching out.
CryptPad 5.2.1 is already a quite old version and isn't supported anymore, it was a good idea to upgrade indeed. Did you followed every upgrade notes from every release between 5.2.1 and 5.6.0?
From the issue you are describing it seems you, at least, didn't followed the ones from 5.4.0 where we dropped Bower support to handle our dependencies management.
Keep us updated!