Cause: probably PHP 7.0
Fixed by going back to PHP 5.6
trac.roundcube.net/ticket/1490544 (http://trac.roundcube.net/ticket/1490544)
When I last updated to ver. 1.1.2 I had a hickup with the rcmail.php file.
On login I would get this error:
Warning: Declaration of rcmail::get_instance($env = '') should be compatible with rcube::get_instance($mode = 0, $env = '') in mail/program/include/rcmail.php on line 30
I manually altered the file so the function declartion matched
static function get_instance($mode = 0, $env = '')
I have noticed that a minor update to 1.1.3 had been released so I updated.
The exact same error message has now returned.
You should open a ticket at: http://trac.roundcube.net/
Will do :D