On PHP7 (Ubuntu Xenial) we see:
[10-May-2017 12:57:43 Europe/Berlin] PHP Deprecated: Methods with the same name as their class will not be construc tors in a future version of PHP; Net_Sieve has a deprecated constructor in /usr/share/php/Net/Sieve.php on line 93 [10-May-2017 12:57:43 Europe/Berlin] PHP Warning: array_merge(): Argument #1 is not an array in /usr/share/php/Net/ Sieve.php on line 312
these are fixed in the Roundcube's fork of Net_Sieve. We should provide that in packaging.