Confluence 9.0 EAP Feedback

Hi,
We are trying to get ready for the upcoming breaking changes that will be introduced by Confluence 9.0. 

Our app is functioning as expected in Confluence 8.8. but we are not able to load the app in Confluence 9.0.0-m09.

The error is the following:

 Detected an error (BundleException) enabling the plugin 'com.balsamiq.confluence.plugins.mockups' : Unable to resolve com.balsamiq.confluence.plugins.balsamiq-wireframes-confluence [307](R 307.0): missing requirement [com.balsamiq.confluence.plugins.balsamiq-wireframes-confluence [307](R 307.0)] osgi.wiring.package; (&(osgi.wiring.package=com.opensymphony.module.propertyset)(version>=1.3.0.21Nov03)(version<=1.3.0.21Nov03)) Unresolved requirements: [[com.balsamiq.confluence.plugins.balsamiq-wireframes-confluence [307](R 307.0)] osgi.wiring.package; (&(osgi.wiring.package=com.opensymphony.module.propertyset)(version>=1.3.0.21Nov03)(version<=1.3.0.21Nov03))].  

We have no direct reference to com.opensymphony.module.propertyset in our app.

The only reference we found in the dependency tree is the following:

 
[INFO] +- com.atlassian.confluence:confluence:jar:9.0.0-m09:provided
[INFO] | +- com.atlassian.user:atlassian-user:jar:3.0:provided
[INFO] | | - opensymphony:propertyset:jar:1.3-21Nov03:provided

We are using the com.atlassian.confluence.user in our app (and it extends com.atlassian.user.User. }}Could this be the problem? As far as we understood {{com.atlassian.confluence.user is not deprecated.
 
Thanks in advance,
Sax