Here is a solution to the problem with JComments 3.0.7:
Joomla version is 3.8.11 and PHP version is 7.1
1. With FileZilla or on another way, rename folder "public_html/plugins/system/jcomments" to "public_html/plugins/system/jcomments1".
2. We can now enter in the backend administrator panel.
3. Download "Jcomments-Recaptcha2-Develop.zip" from https://github.com/exstreme/Jcomments-Recaptcha2
4. Install the file "Jcomments-Recaptcha2-Develop.zip" in Joomla.
5. If everything is OK – delete folder " jcomments1" in "public_html/plugins/system/".
Joomla version is 3.8.11 and PHP version is 7.1
1. With FileZilla or on another way, rename folder "public_html/plugins/system/jcomments" to "public_html/plugins/system/jcomments1".
2. We can now enter in the backend administrator panel.
3. Download "Jcomments-Recaptcha2-Develop.zip" from https://github.com/exstreme/Jcomments-Recaptcha2
4. Install the file "Jcomments-Recaptcha2-Develop.zip" in Joomla.
5. If everything is OK – delete folder " jcomments1" in "public_html/plugins/system/".