If you aint done it yet try checking all external manholes one may be full of s..t, paper, fat, you name it it might be in it. Also are your drains shared? as it may not be your end that's causing it.
Worse things the un-educated put down drains/toilets sanitary towels, disposable nappies, cooking fat. The next worst are large 'Jobbies'
By your description my geuss is it's in one of the manholes and it's banked up in to the toilet even first floor ones where nothing has shown around the manholes such good seals they were, I set up the manhole lid ready to lift stood on a stepladder and....say no more it was everywhere swilling around the stiles of the stepladder. stepped on to dry land then did the business of clearing up.
nobby
September 2008