I think you will although I don't remember for sure. The Opaque Moodle module had some unusual constraints on what was allowed in the identifier strings. I believe it required the initial character to be a lower case letter, but at this point that's just a guess.
look at section 3 "create Moodle quiz
This is a string conatining only the characters [_a-z0-9.], and also
matching the syntactic rules for Java package names. That is, it is one
or more 'words' separated by '.'s, each word matching the regexp [_a-z][_a-z0-9]*
. This identifies the question withing the question bank. For example calculus.diff01a
I wonder if could have used the period . somehow in addition to make it work more smoothly.
What version of moodle are you using? I've had several questions over the last year which bring up things that could be improved about this Opaque server feature. Most of these involve updating the php code in the opaque client which does not appear to be maintained at the moment by Moodle central. I've been considering whether this connection should be replaced by an LTI solution of some sort which would probably be easier to maintain. The same is true for the wwassignment moodle module which is used to connect a Moodle assignment to a homework set WeBWorK. That can be replaced, minus a few features, by LTI. It's possible that with LTI 1.3 any missing features from wwassignment could be provided as well.