SecurityTrimming stops working

I've a website running for a long time. It uses a sitemapprovider with securitytrimming enabled. This has worked for many years without any problems. However, in the last few weeks, suddenly securitytrimming stops working, and all menuitems are shown. When you click on a menuitem which should be disabled because the user doesn't have rights, the user is redirected to the login page, so the security still works.
I noticed that the URL is changed:
http://www.mydomain.com/default.aspx
is changed to
http://www.mydomain.com/(X(1))/default.aspx
When hoovering over all the menuitems, all links also include the (X(1))
In web.config, I configured the forms tag to NOT use the URL
<forms loginUrl="Login.aspx" defaultUrl="Login.aspx" timeout="120" ...