Alec: Looking at the web server logs I do get the following entries every time I get the popup:
ALERT - configured GET variable value length limit exceeded - dropped variable '_uids' (attacker '***.***.***.***', file '/*******************/roundcube/index.php')
ALERT - dropped 1 request variables - (1 in GET, 0 in POST, 0 in COOKIE) (attacker '***.***.***.***', file '/*******************/roundcube/index.php')
ALERT - dropped 1 request variables - (1 in GET, 0 in POST, 0 in COOKIE) (attacker '***.***.***.***', file '/*******************/roundcube/index.php', line 40)
ALERT - dropped 1 request variables - (1 in GET, 0 in POST, 0 in COOKIE) (attacker '***.***.***.***', file '/*******************/roundcube/program/include/iniset.php', line 58)
I suspect perhaps the variable value length limit needs to be increased?