There are many reasons why this would occur. Many of them due to normal behaviour, some of them due to invalid input data, and a rare few caused by 'external' influences.
It uses both (for different reasons). Please refer to the authoritive guide on this matter; the "servers.txt" file included within the AP distribution files for more information.
Meanwhile, if you could give some useful information, such as a URL, and ideally, a demo product, whereby I can duplicate your problem I'll be in a far better position to help isolate the cause of your specific error.
Also, based on nothing other than order of probability, my first suggestion would be to try the enable/disable cURL toggle.
When testing, ALWAYS close and re-open your browser when checking for the effects of this change (otherwise you'll be using cached settings).
Cheers
RodG
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