Redmine

After migration to new server, users haves 500 error when accessing /my/page.

New:
Environment:
Redmine version 2.6.10.stable
Ruby version 2.3.1-p112 (2016-04-26) [x86_64-linux]
Rails version 3.2.22.2
Environment production
Database adapter Mysql2
SCM:
Subversion 1.7.14
Git 1.8.3.1
Filesystem
Redmine plugins:
redmine_contacts 3.4.0
redmine_contacts_helpdesk 2.2.11

Old:
Environment:
Redmine version 2.4.0.stable
Ruby version 1.9.3-p448 (2013-06-27) [x86_64-linux]
Rails version 3.2.15
Environment production
Database adapter Mysql2
SCM:
Subversion 1.8.3
Cvs 1.12.13
Git 1.8.3
Filesystem
Redmine plugins:
redmine_contacts 3.4.0
redmine_contacts_helpdesk 2.2.11

Error message from production.log:
Started GET "/my/page" for 172.30.60.118 at 2016-05-17 14:03:31 +0200
Processing by MyController#page as HTML
  Current user: krzysztof.golos (id=14)
  Rendered issues/_list_simple.html.erb (24.0ms)
  Rendered my/blocks/_issuesassignedtome.html.erb (109.1ms)
  Rendered issues/_list_simple.html.erb (27.5ms)
  Rendered my/blocks/_issuesreportedbyme.html.erb (39.2ms)
  Rendered my/blocks/_issueswatched.html.erb (7.2ms)
  Rendered my/page.html.erb within layouts/base (158.8ms)
Completed 500 Internal Server Error in 169.6ms
ActionView::Template::Error (no implicit conversion of nil into String):
    1: <h3>
    2:   <%= link_to l(:label_watched_issues),
    3:         issues_path(:set_filter => 1, :watcher_id => 'me', :sort => 'updated_on:desc') %>
    4:   (<%= Issue.visible.watched_by(user.id).count %>)
    5: </h3>
    6:
    7: <% watched_issues = issueswatched_items %>
  app/views/my/blocks/_issueswatched.html.erb:4:in `_app_views_my_blocks__issueswatched_html_erb___2499037382860342811_70277487537940'
  app/views/my/page.html.erb:29:in `block in _app_views_my_page_html_erb___2611345217716520421_70277481863500'
  app/views/my/page.html.erb:26:in `each'
  app/views/my/page.html.erb:26:in `_app_views_my_page_html_erb___2611345217716520421_70277481863500'

Can anyone help?