When we are publishing OWA, or every web service through TMG and we are willing to make use of FBA we have the chance to change our password through the FBA web form. However this step is not always as straightforward as it seems and there are some possible pitfalls in the configuration on the TMG or on the DC.
In this article we want to provide some guidance how to troubleshoot these problems and also how to identify specific issues that can prevent the FBA changing password from working as it should.
Of course the first point when you see the error message is to check if the “complexity requirements” are really met and if the user who sees the error is the only one affected by this issue.
Read more at source: http://blogs.technet.com/b/isablog/archive/2012/05/07/tmg-2010-fba-troubleshooting-the-change-password-feature.aspx