So the problem, as we suspected, was caused by a regular expression

It's been fixed in the main code base for some time now, so the next release of Avactis will work fine in this regard.
Details:
In the following two files:
avactis-system/modules/customer_account/abstract/validator.php
avactis-system/core/validator.php
...the
$email_rx variable is instantiated. It contains a not-so-perfect regular expression string. Put the correct line there and it'll be OK.
The correct line of PHP code should be the following:
$email_rx = "/^[a-z0-9]+([\.\-_][a-z0-9_-]+)*@([a-z0-9]+(-?[a-z0-9]+)+\.)+[a-z]{2,4}$/i";
Sorry for the trouble.