Where to download older versions?

If you are looking for 4.7 version of the plugin : https://downloads.www.ads-software.com/plugin/contact-form-7.4.7.zip

Thank you!

try rebuilding permalinks, worked for us.

Rebuilding permalinks is unlikely to work if you are blocking the REST API -anybody that has a reasonable firewall like Ninja Firewall will either need to downgrade and test or disable that option in their firewall plugin. Likewise anybody who has an htaccesss rule blocking it. This type of thing should have been spotted before release. There are tens of security articles about exploits on the REST API.

Downgraded to CF version 4.7 and all appears well.