Suddenly access denied on some teamsites for some users

On our SharePoint farm we have multiple teamsites. Suddenly some users get access denied on some teamsites. When I give them read permissions on the parent site it works again.

I am aware that this happens normally when the site loads some content from the parent site, but it happened at a single blow without changing anything. There is nothing obvious that the site has to load from the parent site because they should be independet and it happens totally randomly. Some users with same permissions have access, some not. Even if both users don't have permissions for the parent site.

ULS log and IIS log don't tell something special. Just access denied and redirect to error page.

I don't know if it has to do with the same error but suddenly we are not able anymore to save a site as a template.

Thanks,

Stefan...