Water backup in condo

[rp911]

We're having a weird problem in our condo; in the past, there have been a couple of times when waste water (brown in color) has backed up into the downstairs bathtubs of everyone's unit. We've had the main line cleared to the street (believe there is a dip in the pipe under the slab) and the problem has abated for months. To avoid digging up the lobby, we put the building (four units all townhouses) on a 90 day schedule of having the line cleared as precaution.

Now I'm having a problem in my unit; I'm getting bubbling in my downstairs toilet occasionally, and a little clear water appeared in my tub, maybe an inch or so. We had the line cleared a week ago, but last night the toilet was bubbling again, and I got about three inches of water in my tub. Turns out my neighbor was emptying his bathtub and running his washing machine at the same time.

This doesn't seem like it's actual sewage backing up from the street, it seems like we're having trouble draining water from all the units, but this time it's only my unit where the problem seems to exist.

We're planning on getting a camera into the line tomorrow, but I'm wondering if there are other potential causes to think about. All the roof vents appear to be clear, at least visually.

Any ideas appreciated!