I had the exact same experience but I posted it in the old beta test thread and should have posted it here. Below is an editted exerpt from that post and subsequent update but the upshot is that the new app either can not handle hundreds or thousands of recordings or there was one or more problematic recordings it could not handle and when I removed enough or the problematic ones it started to work.
Editted excerpt from my previous post with additional information:
I just installed the new firmware on my main Tablo that was not part of the beta program with mixed results. I have a problem trying to use the Roku Gen4 app with my main Tablo. I tried this on two different Rokus with similar results. They both initally launched, on one Roku I went to the library and after a few seconds the app closed and whenever I try to relaunch (even after restarting the Roku) it will try to load but closes. On the second Roku, I went to the Live tab and started configuring the guide view and all of a sudden it crashed too and behaves like the first Roku when I try to launch the gen4 Roku app again. No combination of removing and re-adding the app and restarting the Roku improved the situation. My main Tablo has almost 2000 recordings. I’m not sure if the gen4 Roku app is having trouble handling this many recordings, perhaps similar to the ONN issue previously reported.
Update: I decided to see if reducing the number of recordings would help the situation and the short answer is Yes. Over the weekend I deleted what I no longer deemed necessary and transferred hundreds of recordings to my Plex server via Tablo Ripper. When I had only a few hundred recordings left on my main Tablo, I could suddenly use the Roku gen4 app to access it. This was consistent across multiple Rokus so the problem was not tied to any specific Roku device. Does anyone else have hundreds (or thousands) of recordings and have you seen a similar problem? FYI - This is a problem only with the gen4 Roku app, the gen4 iOS (iPhone) and Android (Samsung Tablet) apps worked fine with ~2000 recodings.