How to recreate:

• Create a new Maven 2/3 job.
• Add the flexible publish post-build action
• Add one condition action (I used xUnit with jUnit on Files Match)
• Save
• Edit again, and add another conditional action (I used the JaCoCo publisher with Files Match).
• Save

I get: Failed to instantiate class org.jenkins_ci.plugins.flexible_publish.ConditionalPublisher

I expected it to save.

Ciao!