Hi Scorpiankh,
had the same issue right now after upgrading to 2.4.1
Following steps helped me out:
Check in the Databasetable "SiteTree" that all entries have a ClassName defined. If not, add the corresponding ClassName manually.
In mysite/code, check that no "CalendarDateField" is defined in any function (e.g. getCMSFields). If so, just rename it to "DatePickerField".
Log into the CMS, go to the Behaviour tab and check if all pages show the appropiate Pagetype. Finally, make sure that all pages have some content. The pages I could not save did not have any content. By just including "Lorem Ipsum" to the content area, they saved without problem.
Seems that on some point upgrading to 2.4, erases some ClassName fields in Databasetable and causes some Pages to show up as "Pageytpe:SiteTree", which should not be.
Hope this helps.