Can no longer login - Admin settings ineffective - All (multi)sites affected + PHPMyAdmin

I submitted this issue to Authcache too, as I am not sure which - if either - module could be involved.

While "playing" with Drupal caching modules Boost and Authcache, I suddenly notice some strange behaviors
- Admin settings were not taken into account: i.e. unsetting a module did not change anything, manual backup and migrate did nothing and so on
- However, not creation / update worked correctly
- If changing the settings in the database, the site were Authcache was installed did not take care - i.e. I unset Authcache and Boost in the database, but the modules stay active on the module list as well as in reality. The sites without authcache were actually taking the new settings that couldn't be changed from Drupal UI
- When disconnecting from one site, there is no way to reconnect - changing the browser did not help, I stays anonymous for ever.
- no error appear in any logs, no apache connection attempts are recorded, no error displayed in the browser, it stays silently on the login page.
- Even PHPMyadmin is affected: I can no longer manage my databases from this app. As for Drupal sites, I stay on the login page without any error.
- As this was a multisites install, I tried to create a new, stand alone site. The last install wizard step failed to complete - it kept asking for the site email, admin login... whithout going further. The site was created, but again no way to log in.

I am not sure Authcache or Boost are responsible for this, but any help will; be much appreciated.

By the way, are there any means to clear caches from outside Drupal? DB tables to truncate, file to erase???

Thanks