Currently I maintain 10 different Drupal 7 web sites. They were designed and built by 3 different developers. These sites are running on 3 different platform types. Some are on a dedicated server, some on a VPS server and some on Pantheon. The dedicated and VPS servers are running Apache 2 and Pantheon uses Nginx. They are on various versions of the Drupal core, between 7.43 - 7.56.
All of these sites are experiencing the same issue, at some point the main navigation menu which displays on all the site pages just stops displaying. When this happens there are no errors in the Apache or Drupal logs.
On the dedicated server and the VPS servers clearing the Drupal cache (sometimes more than once) will bring the menus back. On the Pantheon servers, clearing the Drupal cache (even multiple times) does not work. The only way to do it is to clear the cache for the site in the Pantheon dashboard for that particular site.
The only common things between these sites are:
- There is nothing "special" about these menus. They were all designed using the standard Drupal utility in the Structure menu
- They are all sub-themed using the Drupal Bootstrap 3 as the base theme, currently at 7x-3.13
Google hasn't helped so I'm wondering if anyone has seen this error and if so, where I can start looking to correct the issue.