Curl and SSL "unable to get local issuer certificate"

Just another (hopefully) helpful post for anyone that is having trouble when performing a network update.

I was doing a few adjustments to my multipress installation and noted that I suddenly started to get an error when using the "update network" feature. The error was:

Warning! Problem updating Your server may not be able to connect to sites running on it. Error message: SSL certificate problem: unable to get local issuer certificate.

After looking at the error logs (which reported nothing) and some other things, I found that the problem was being caused by curl (the command line tool for transferring data with URL syntax)...

To be honest, I have not used curl much (except by just installing it and letting the site do what it needs to do) so I can't give much advice about configuring it with SSL. In the end I didn't need curl anyway so removed it from the server and the errors stopped occurring.

There's info about curl and SSL at if you really need to use it and having these problems.