HTTP (not HTTPS!) connections are being refused over mobile connection

[xarx]

Environment

LG D620R G2 mini with Android 4.4.2, rooted with TowelRoot application; ABP 1.3 build#359 and #368; SuperuserSU Free 2.40 used for granting root access to ABP.
Tested with the built-in "Internet" browser and with the Dolphin browser.

How to reproduce

In the following, I'll describe steps with ADP in root mode.

1. Turn on mobile data connection, ADP filtering must be on.
2. Goto ​http://www.amazon.com/, ​http://www.google.com/, ​http://www.microsoft.com or any other site over ​http:// protocol. The following error occurs:

Error: 500
Got the error: Server Error
while trying to obtain ​http://www.amazon.com/
Connection refused

3. Now try to access the https version of the same sites:

​https://www.amazon.com/, ​https://www.google.com/, ​https://www.microsoft.com - they work!

4. Over wi-fi connection, both http and https connections work.

Observed behaviour

Seems to be the same problem as reported here:
​https://adblockplus.org/forum/viewtopic.php?f=15&t=25038

I'm not sure that my results with ADP in non-rooted mode are relevant, or whether the inability to connect over mobile data connection is caused by another problem. Over mobile data, http pages return the same Server Error as above, but https pages return ERR_TUNNEL_CONNECTION_FAILED. Over wi-fi, both http and https work.

Expected behaviour

ADP should be able to access both http and https pages over mobile data, as it does over wi-fi.

[xarx]

I've tested several ABP builds since build #318 till build #368, and they all exhibited the same problematic behaviour.