The “dev”-mode was there ever since the first public FPOS-Version.
[ I’m pretty sure about this, because the FP-Update-Request, which I sniffed in this time was http://ota.fairphone.com/FP2/updater.zip?&ota_v_n=<version>&beta=0&dev=0, the dev=0 means that the developer mode is off. For some versions now, the updater uses https, so the developer-mode is pretty useful for sniffing update-requests 
]
@M_Dammer:
I agree with you, that this is only a workaround, but I also think that there is no bug. I think the updater on FPOOS and FPOS are pretty much the same code, and I just wireshark’ed the traffic of my phone:
I didn’t find, that the phone automatically checked for an update on connecting to my WLAN (could be a timer, which checks only once in x hours or so…), but as soon as I opened the Updater, I saw encrypted traffic to ota.fairphone.com (which, btw, CNAMEs to ba.fairphone.com).
Please note the updater was closed before, so just tap on the square and swipe it to one side, if it was open before.
Edit: Another Idea: You wrote about the clock bug, but you set your time correct?! If that isn’t the case, any HTTPS-Connection will fail, so the update-request will always fail unless you manually set it to http. You can easily check this by visiting any encrypted website with your browser (e.g. https://google.com). If there is no error, everything is fine.