Redmine issue details “page not found” after update

[alexander-st]

I upgraded my Redmine installation from 2.6.0-2 to 3.0.2-0. Everything seems to be in place. All users, issues, wiki pages are there. But if I want to see the details of an already existing issue, I get:
Page not found

The page you were trying to access doesn't exist or has been removed.

In the following a snippet from the production.log

Started GET "/redmine/issues/1161" for 46.244.87.237 at 2015-04-28 21:49:07 +0300
Processing by IssuesController#show as HTML
Parameters: {"id"=>"1161"}
Current user: alex (id=1)
Rendered issues/_action_menu.html.erb (13.0ms)
Rendered issue_relations/_form.html.erb (4.0ms)
Rendered issues/_relations.html.erb (25.0ms)
Rendered issues/_action_menu.html.erb (4.0ms)
Rendered issues/_form_custom_fields.html.erb (2.0ms)
Rendered issues/_attributes.html.erb (71.0ms)
Rendered issues/_form.html.erb (185.0ms)
Rendered attachments/_form.html.erb (12.0ms)
Rendered issues/_edit.html.erb (240.0ms)
Rendered plugins/redmine_watcher_groups/app/views/watcher_groups/_watcher_groups.html.erb (8.0ms)
Rendered plugins/redmine_watcher_groups/app/views/watcher_groups/_watcher_groups_sidebar.html.erb (83.0ms)
Rendered issues/_sidebar.html.erb (93.0ms)
Rendered issues/show.html.erb within layouts/base (546.1ms)
Completed 500 Internal Server Error in 1578ms (ActiveRecord: 139.4ms)

ActiveRecord::RecordNotFound (Couldn't find all Watchers with 'id': (all, {:conditions=>"watchable_type='Issue' and watchable_id = 1161"}) (found 0 results, but was looking for 2)):
lib/redmine/hook.rb:61:in block (2 levels) in call_hook'
lib/redmine/hook.rb:61:ineach'
lib/redmine/hook.rb:61:in block in call_hook'
lib/redmine/hook.rb:58:intap'
lib/redmine/hook.rb:58:in call_hook'
lib/redmine/hook.rb:167:incall_hook'
app/views/issues/_sidebar.html.erb:21:in _app_views_issues__sidebar_html_erb__1047423174_68682276'
app/views/issues/show.html.erb:146:inblock in appviews_issues_show_html_erb___242619375_44094252'
app/views/issues/show.html.erb:145:in _app_views_issues_show_html_erb___242619375_44094252'
app/controllers/issues_controller.rb:118:inblock (2 levels) in show'
app/controllers/issues_controller.rb:115:in `show'

It seems to me that the issue detail page is not working because of the plugin. I unistalled the plugin and then the detail pages are working again.
What can I do?
Thank you for your help!

[maxrossello]

Redmine 3 is relatively new and not all plugins support it yet. Stick with 2.6.x if you need this plugin, for the moment, and monitor forks for new contributes since this branch seems idle

[alexander-st]

Thank you for your answer.
It looked to me that the information of the plugin was not included in the DB dump...