@hschinnenburg The problem is with the entry of the attribute value, not the replacement.
When someone enters their company that includes an
& then it is stored as
&. If they then edit their preferences the value is encoded again to
So it is a bug in how the attribute values are processed.
One workaround is for you to look at the database to see any bad values and change them there.
You could also use the Subscribers plugin to search for any values that include, say, “amp”. Follow the “find plugins” link on the Manage Plugins page to see the available plugins.