The error message for users is really bad. As far as I can tell there is only one error message possible at all: ~ "Signing is not possible. Invalid password for the certificate or unable to write to FILENAME"
This is obviously Okulars catchall "General Error" and not only used in the two (totally unrelated) cases it mentions. I saw it also when the root certificate chain of the chosen signature certificate was not trusted, in which case you are not asked for a password at all.
Needless to say this is very confusing for users and a nightmare in support.
Please find a way to give precise error messages.