IPN issues - again "PayPal Instant Payment Notification Warning"

When I first set Membership up last year I began getting error messages from PayPal warning me of the IPN issue. I asked on here and after some time, was told the settings were ok and it would be fine.

Some time later new members stopped being upgraded and I had no idea why. Eventually, again after asking for help on here, I realised that the IPN had been disabled, and it looked like I had done it by mistake but I know I hadn't. But still once re-enabled members were once again being upgraded.

Over the last few months it happened again - members stopped being upgraded, again I came on here and got not solution. Turned out the IPN setting at PayPal had once again been disabled. It looks like because of this error, PayPal have disabled it.

So I re-enabled it, and now I'm again getting the IPN warning emails from PayPal, so I know at some point the IPN will be disabled and my new members will not be upgraded automatically.

So, there is clearly a problem somewhere with the set up but I can't find it. I have read through all the other IPN threads on this forum and there are not many solutions at all, any potential solutions have not worked for me:

Permalinks are set to posts.
My web host (heart internet assures me it's not their problem).

I would love some assistance with this.. I'm not the only one to experience it so it really does need to be fixed.

Please help.

Thanks
Jane