This answer i got from support (via mail):

 

We can promise that we didn't disable the ONVIF function of the camera. We have no reason to do this.  

This may be caused by some unknown incompatibilities when the camera upgraded to the firmware 1.3.9 in the specific environment. Actually, we failed to reproduce the issue at our end and we haven't found the root yet.

So, we sincerely hope you could help us collect the diagnostic log from the camera to troubleshoot the issue further and resolve.

We would like to help you solve the problem.

Your cooperation and understanding will be appreciated. 

We look forward to hearing back from you. If you have additional information or feedback to provide, please feel free to let us know.

  @firefox7518 I noticed an issue with my cams not working in Blue Iris.

However they were blocked from internet access so couldn't be firmware update related.

So I let them connect to internet and connected Tapo App to them fine.

However I then updated my firmware which won't help with pinpointing.

But I do remember changing some router settings recently.

With the mention of ARP in here I went and checked my router.

In my Unifi router I have a setting called 'Proxy ARP'.

I enabled this and my cams work again via ONVIF in Blue Iris.

This might not be related to the firmware and me just not noticing them not working after playing with router.

Thought it might be worth mentioning though.

  @Paul_Rickwood 

I mentioned proxy arp before as well but I don't think that's the exact cause.

I think sometimes they boot up and work fine and other times not. 

  @Solla-topee 

again, that's hard to believe you guys continue to claim this, when we are all saying the same thing. Most of the replies on here are clearly from technically aware people who know what they're doing.

I myself work for a major Australian company in tech support myself, and I would never answer people this way or let an issue go on like this. It really reflects poorly on tp-link and the word is already out there about this update breaking things beyond this forum. 
 

I've ticked all my boxes in my profile, I guess this what is you are referring to, I happily await contact from your tech support team, happy to give out max addresses and whatever else you need. 

I'm facing exactly the same problem.
I have more than 10 tapo cameras on my network and I only updated one of them to version 1.3.9 and now it no longer works in iSpy agent.

Is there no way to downgrade the version?

In my case, the camera was working correctly, along with all the others, and I just had to click on the update option and when it restarted after the update, it no longer worked.

  @Solla-topee I am facing the same problem.

The camera was working normally and as soon as I updated the firmware, it simply stopped connecting to iSpy Agent.

There were no configuration changes on the network. The only thing I did was update the firmware.

My entire network is from TP-Link and I have several other cameras working normally on my network.

I don't know if you are doing the appropriate tests but it seems that you are not testing with the C200 on iSpy Agent because absolutely everyone on this forum is complaining about the same thing.

I've also still got the same issue - multiple V1 cams working perfectly, V3 cams refuse to connect to anything but the Tapo app.

Tp-link router - other tapo cameras (V1 & V2) working fine

I've just sent off a detailed email with all my settings / logs collected to Graham your senior engineer.