SI CAPTCHA: Error reading captcha cookie

Although I’m using Si Captcha for at least 2 years and never having any problem, since last update it doesnt accept comments anymore: “Error reading captcha cookie”
First I thought those people use computers behind a firewall where cookies are disabled. But no, when trying 8 different browsers and all having cookies enabled it still says, error, can’t read cookie.

Meanwhile I disabled Si Captcha because 0 comments are working.
On the other hand I’m getting now dozens of spam comments what is also no permanent solution. Thanks for any suggestion!