In the phpMyAdmin or whatever your database editor for the localhost installation is, go to your phpbb_config table (or whichever table name prefix you're actually using, instead of "phpbb_") and find the
form_token_sid_guests row. Change the config_value field for that row from the default of "1" to "0". (The "is_dynamic" field will already be "0", and should remain that way.)
This disables the "Tie forms to guest sessions" in the ACP Security Settings section. If that does help you work around the problem, it means the session ID was changing on your logged-out sessions, for reasons I don't actually know how to explain yet.
Note this does assume you're using the current phpBB 3.2.7. Getting "invalid_form" at login on phpBB 3.2.6 could have other causes.
Community Team Member Note Proceed with caution and only if you REALLY know what you are doing.
Last edited by HiFiKabin
on Wed May 22, 2019 4:04 pm, edited 1 time in total.
Reason: Caution note added