Greetings!
I am sorry to hear that.
From my point of view, it seems that the SSL certificate expired and has to be renewed once in a while for your domain.
Kindly, may I ask have you checked within the steps from below articles regarding this error showing up for your Website?:
May I ask, before moving to Cloudflare, have you had an SSL certificate at your host/origin server for your Website and was it surely working over HTTPS connection? (as you stated, it obviously did)
Furthermore, may I ask what SSL option have you got selected at Cloudflare dashboard for your domain at SSL/TLS -> Overview tab? Is it Flexible, Full SSL, Full (Strict) or something other?
I am not sure about 1and1 and their process of issuing the SSL certificate for a domain, but you could temporary switch the A type DNS record for yourdomain.com and www.yourdomain.com (and other if needed) from 
to 
.
Wait for a day or if you can somehow check if the SSL is still a valid one and covering your domain. In case if needed, try to renew your SSL certificate through their interface.
Maybe a below links could help a bit too in this process:
- Buy SSL Certificates at a Reasonable Price & Secure your Hosting | IONOS
- SSL Certificates - IONOS Help
After success and your new SSL certificate is generated by 1and1, switch back from to to make sure your Website is proxied by Cloudflare, therefore re-check and select Full (Strict) SSL as an option at the SSL/TLS tab from Cloudflare dashboard for your domain.
Otherwise, you could temporary turn on the Pause Cloudflare on this site option to make possible for the SSL certificate to renew.
Test out if you can get an HTTPS connection and if the SSL certificate is valid using for example below online tools:
Here is a way to re-check if you correctly setup the SSL for your domain with Cloudflare:
If any other issues appear, follow the needed steps for troubleshooting from article below:
Regarding available SSL options at Cloudflare dashboard, check here:
In case you do not manage to renew your SSL certificate or you do not have an SSL certificate at all, you can use Cloudflare SSL, if so, kindly make sure you follow the instructions as follows on the below article to setup an SSL certificate using Cloudflare CA Origin Certificate:
Last but not least, kindly have a look here for more information regarding correct SSL settings:
Just in case, may I ask was your Website/application working over a compatible and supported port when using cloud (proxy mode)?
The compatible and supported port list (having the proxy mode enabled) at Cloudflare can be found here:
https://support.cloudflare.com/hc/en-us/articles/200169156-Identifying-network-ports-compatible-with-Cloudflare-s-proxy