Let us know what you find out. Seems to be an issue at the router level mostly.
Yes. Although we all understand the problem in this forum thread. D-Link themselves seem to have trouble reading (both my messages themselves and their inability to read this forum thread). Currently battling with them to try and get them to understand the root cause. At the moment they are simply stating that they've tested it with their own FTP service (from some dlink device) and cannot replicate the issue, almost (but not quite) therefore insinuating that all is well. They are asking me to test with another FTP service, despite my previous correspondance informing them of exactly that (2 tests with VSFTPD and Filezilla Server and also obviously my previously working router). I've also directed to this thread for the 2nd time, hopefully they take the recommendation and actually read it.
Apologies for rant-level of tone, not directed at you guys obviously, it's to D-Link who might hopefully now be reading this. As much as we all love tech, etc, I'm sure the majority of us are busy and time is relatively precious. Repeating facts to D-Link technical support, who by their very nature should be the ones who have the correct problem-solving mindset and analytical skills more so than the average consumer (OK, I'm a software dev but not quite the point!), is not a constructive use of my time. More so especially as they have identified and solved this problem once already with a different camera (also, potentially manifesting itself as a camera-wide problem). You guys are great though obviously!
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Will update as time goes on.