attention required: cloud flare

Recently while accessing some websites with my MacBook Pro running OSX 10.11.2 I started to be redirected to a Cloud Flare page asking me to enter a captcha. I thought first that this was a problem with the Firefox browser, but Safari brings the same result. I tried to access the same URL with my iPad (Safari and Firefox) and it brought the same result.

Interestingly, using an Android phone on the very same WiFi AP does not bring up the Cloud Flare page and connects directly to the URL without trouble.

So, the problem seems to be limited to Apple devices.

Does anybody know what the problem might be? It is actually very annoying to have to enter a captcha which is very difficult to read everytime.

The Cloud Flare page suggests to proceed with a malware scan of the system. I did it with Bitdefender which does not turn up with anything suspicious.

Thanks in advance for the help.

JP