HTTP AUTH (auth_pam module) Can't initialize threads: error 11

[E.B.]

> Using the auth_pam module to implement HTTP AUTH:
>
> https://github.com/stogh/ngx_http_auth_pam_module/
> 
> Once in a while authentication seems to stop working across all browsers
> and users. The error that shows in the Nginx error log file when a browser
> tries to authenticate is:
> 
> Can't initialize threads: error 11
> 
> (Verbatim, the error has no timestamp or anything else)
> 
> Restarting Nginx fixes the problem for some time (days?).
> Next time I'll try a reload instead.

Reload also fixed the problem.

But its not possible to use software that breaks once a day.
Can anyone please help?

> Searching for that error doesn't turn up too much, except that it
> might be a MySQL error(?)
> 
> Can anyone help? Is the author Sergio on this list?