apt-get install php8.0-mbstring
I would like to greet you dear colleagues!
I want to describe all the actions in detail because our joint research has already brought gifts to know the cause of the error.
And according to the results of the cause of this error, we used the php extension update advice
The system returns a message that the update has already been installed
Try again to go to the roundcube interface
apache2/error.log
I summarize with deep respect
I bow to respected colleagues and perhaps someone will have ideas for solving the source to order.
I will thank you, and perhaps many more who may have the same problem will also thank you many more times and take off your hat.
Respectfully guys
error.log in /var/log/roundcube/error.log