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

[E.B.]

Hello,

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.

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?