That seems strange. I don't have CC 2015 at work to test, but if I have time tonight when I get home I can try to recreate your situation.
Does it happen to you in a more simple composition? Like, if it's just one layer of footage and an adjustment layer with Levels, does it still act strange? If so, that's odd; PLEASE file a bug report! If not, what other effects are in the badly behaving composition?
Also, what OS are you using?
Tried more tests. Strange results.
The comp that was having the error would scrub live with a #2 adjustment layer with Hue effect. But with #1 adjustment layer above that with Levels effect it will not scrub live.
Then I tried a new comp. Works fine with an adjustment layer above video.
I think this has to do with some strange combination of effects and layer that AE does not like.
Also rendered finals of those comps, all finished ok but with many errors....
OS X 10.10.5
That error message seems to be about AE's cache not finding it's own files. Is there a chance that some software on your system is cleaning up or performing maintenance on the hard drive that contains your cache when you're not looking?
Do you get these error messages if you clear your cache first?
As to the update being borked in your project, that's really odd. Any chance you could share your project file that's got the issue?
I have cleared the cache and relocated all of the cache folders after doing disk utility maintenance.
Scrubbing LIVE UPDATE is now behaving correctly again but only in the NEW COMP. The old COMP is still VERY slow to respond to clicking the Levels adjustment layer on and off. It does not scrub or Live Update either.
I can provide the AE project.
You can send me a direct message with a Dropbox link (or something like that). I can try to look at it tonight when I get home.