Issue: Headlines fail in 4.1.7

I am unable to reproduce this issue on the demo using 2.2.13. Please send me a link to a wiki page that has this issue.

Since this was resolved by disabling and reenabling VaultWiki on your site, this leads me to the following conclusions:

- All custom BB-Codes provided by VaultWiki were affected, not just the H tag. This is because the vw_varname column was missing from the bbCodeCustom cache. With this column unavailable, there was no way to match custom BB-Codes with the correct function that renders each one, including the one for the H tag.
- In order to get bbCodeCustom cache to have this column missing, the cache must be rebuilt while VaultWiki is disabled, or the VaultWiki extensions to either the BB-Code entity or the BB-Code finder are not being executed.
- A normal way for the cache to rebuild while VaultWiki is disabled is by upgrading XenForo. However, VaultWiki catches this situation because it ALWAYS verifies that the cache contains vw_varname before trying to use it. This worked correctly on the demo board so I was unable to reproduce the problem.
- When VaultWiki detects that the cache is missing columns, it will rebuild it before using it. So, at least one of the following must be true:

• The BB-Code entity was not registering the vw_varname column at all (possible add-on conflict)
• The BB-Code finder was not including custom VaultWiki BB-Codes (vw/vwcustom) in its whereAddOnActive check (possible add-on conflict)

In both cases, an add-on conflict could have randomly resolved itself when re-enabling VaultWiki if certain class extensions have the same execution order. In this case, the problem might randomly reappear when any add-on is disabled/enabled/installed/upgraded.

I have also considered the possibility of a race condition when upgrading a busy forum, but I don't think it would be possible chronologically for this to be the cause in this case.
Or an out-of-date PHP opcache, but I don't think this is possible either chronologically, unless certain VaultWiki files were used from a very old version when it was upgraded and the warnings about incorrect file hashes were ignored, and the new versions of those files were finally uploaded after XenForo was upgraded.

Unfortunately, since the problem is now resolved on your site, and I can't reproduce there or locally, and I can't review the code of your various other add-ons, I can't narrow down the problem any further.

If you encounter this problem again, please respond to this report rather than opening a new one. For now, I am closing this as Cannot Reproduce.

Thanks for looking into it.
It may be related to this issue:
https://www.vaultwiki.org/issues/6374/
After which I upgraded VW by first disabling the addon. Then upgraded the software trough XF addon upgrade.