Basic HTTP auth prompting too many times

[E.B.]

Hello,

I have set up HTTP auth using the auth_pam module (although
I'm not sure that module is the problem - it might be nginx problem).

https://github.com/stogh/ngx_http_auth_pam_module/

All works great for a while----

After some time, browsers begin to prompt for authentication over
and over again (I guess once for every image, stylesheet, script, etc?).
Or maybe it is prompting because the credentials failed, but I don't
think so because if I hit cancel/ESC over and over again, I can use
the web page (I'm still authenticated), but none of the images or
scripts have loaded.

The logs don't show any indication of what caused the prompts to
start showing. Previously, I saw this error (in nginx's error log)
associated with the situation:

Can't initialize threads: error 11

This looks a little like a MySQL error (I use pam_mysql behind
auth_pam).I don't know if there is some bad code in auth_pam
causing this(?). Restarting nginx fixed the prompting in this case.

However, today, the prompting started again and the above error
does NOT appear. I don't see any errors. Browser doesn't matter -
tried it on firefox, mobile, whatever.

I will try to test with just the built-in basic auth but that's not a long
term solution for me, I need pam/mysql behind the auth (lot of virt
users).