Cannot get response from external service API

[Denix19]

Since a few days I am not able to receive reponse from an external service API that I used for months without issues. I contacted the service administrator and we tried to understand what was the issue. From the service side there was no problem: my request is received but I cannot receive the API response.Given that I did not make any changes to my site lately, we supposed the issue is due to cloudflare. In fact the same issue occurred for other 2 different services: they worked fine for months, then I could not be able to receive their API responses overnight.

I searched for someone here having a similar issue and the following is the only topic discussing an issue similar to mine:

cURL request to zip triggers managed challenge, cannot bypass

Since some time recently, Cloudflare has started to challenge PHP cURL requests to zip files on our server, effectively blocking the cURL. This does not happen from all locations, but from many. We use cURL for an application installer, which now does not work for many clients because of this. The firewall event lists as “Managed Challenge”, and there does not seem to be anything we can do to prevent this behavior (for example for requests to *.zip files). I tried to add a firewall “allow” rule…

However, I already tried the solutions discussed in the above topic (I inserted an ‘allow’ IP access rule for the service IP), but nothing changed. Before trying with this solution, I also tried to disable every firewall rule and every IP access rule. I could not receive the API response even in that case.

Is there anything else I can try to solve this issue?
Thank you.