strange problem

I have a strange problem when trying to update or publish content. When clicking on update or publish I either land on the home page with a “page not found” error or I land on the main posts or pages screen. In either case, the changes I make are not updated.
I tried:
1. resetting permalinks
2. deactivating all plugins
3. changing themes
4. reinstalling wordpress
5. deleting the cache
6. different browsers
but nothing worked.
So I then tried another computer and I was able to publish and update content without a problem.
So I then:
1. did a virus/malware scan
2. uninstalled any recently installed programs
3. cleared the dns cache
– but still have the same problem.
After playing around a bit I also noticed that if I enter just 2 lines of text, or less, I can publish that content but anything over that and I have the same problem referred to above.
Everything else in my wordpress admin is working as it should and I can make and save changes to widgets, menus, etc but not posts and pages.