It works in 5.1 with this change

[R-i-c-k-y]

in redmine_issues_tree.rb you added:

module RedmineIssuesTree VERSION = '0.0.15' end

so you have to remove version.rb from lib/redmine_issues_tree

[Loriowar]

It might works, but for Redmine 5.1.0 need to create new branch and properly compare and migrate all modified partials/views. In such approach, this will be checked plugin release compatible with the new Redmine version. Untill that you can use different workarounds, but no guarantee of correct work for all use cases.