I have also seen the "POSIX error: Too many open files" with SP17. The solution was to simply quit and restart OW. It always happens after OW has been open for some time and I'm opening a bunch of background tabs as I scroll down a news site or something.
Similarly, I have found that in all sneaky peeks so far that if I leave OW open and idle for awhile (4 or 5 hours) that when I start browsing again it will sometimes fail to load images and / or css stylesheets correctly. The text all loads up fine, but most (not all - most) of the images are missing (and will load if I right click -> Load image) and the layout is messed up and arranged serially as if the css stylesheet was missing. I usually notice it when I visit
http://www.cbc.ca/news/, but it happens with all sites once it starts happening. Also, once this happens, the Omniweb Icon in the dock disappears (goes completely transparent). Quitting Omniweb and restarting always fixes the issue, except for the dock icon, for which I have to remove OW from the dock, and then drag it back in again. I've been having the exact same problem since I started toying with the sps back at sp 10. I'm running 10.4.7 on an 2 GHz Intel Duo 20" iMac.
OW 5.1.3 works perfectly in both of these regards.