405 error in Jira 9


I am building a plugin to be used in Jira 9, and it works fine on Jira 8(local environment), on Jira 9(custom environment) I get a 405 error.
I tried the solution: How to work around the @SupportedMethod problem? - #11 by JohannesKstle, but it failed on my plugin.
How can I test the plugin in Jira 9 when developing?

Appreciate any help on resolving the issue.