I finally heard back from Zapier and they suspect that the issue is due to provisioning inconsistencies in the server pool used by Backendless. They asked me to check with you to ensure that all of the servers in your server pool are provisioned correctly and that TLS support is consistent across the server pool.
Please let me know if there is anything else you would like me to ask for. I’m still unable to resolve this intermittent issue.
I checked from our side only one server is engaged in SSL handshake, there is also a spare server that is not used. I checked all the servers that can participate in this, everywhere the certificates respond equally correctly.
What exactly did Zapier’s support say about the verification from an independent source, which is available at this SSL Server Test: bijouwash.backendless.app (Powered by Qualys SSL Labs) link?
We checked several times and the answer of this service was always correct.
It is possible that some of the TLS modes are not supported bt Zapier, this needs to be clarified.
I looked at the reports described. I see that they are the same, but I have one guess, that’s why I made some changes in the priority of certain encryption algorithms that are supported in TLS 1.3, I made changes. Could you check from your side or indicate how to check that your logic works?
We also saw a similar issue when connected to an office network, and it was due to the firewall injecting a different certificate. Seems like it is unrelated to the issue here, but that may be helpful for you in tracking down what is going on at your WeWork office.