Existing user can't login if changes occur with Legal module

I have installed the Legal module and created a T&C page. New users can create an account through this module, but current users who have not confirmed the mandatory legal terms are shown an error message instead of being asked to accept the terms.

This is all they see in the modal window (white background, no style, only unformatted text with no link, button or any HTML other than all the @import rows):

An AJAX HTTP error occurred.
HTTP Result Code: 200
Debugging information follows.
Path: /?q=system/ajax
StatusText: OK
ResponseText:
Terms and Conditions | Secondwheel.org  
@import url("http://mysite.org/modules/system/system.base.css?m5npz4");
[...]
@import url("http://mysite.org/sites/all/themes/omega/omega/alpha/css/grid/alpha_default/normal/alpha-default-normal-16.css?m5npz4");
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