this 'MooV' file is damaged or unsupported. - AE can't open mp4 files which were once "online-only"

[coolguy2020]

In Dropbox you can set some files or folders to be 'online-only', which makes them visible in Finder and apps but physically they're not available to open unless they are made 'available offline' and downloaded.

For some reason AE has problem opening mp4 files which were previously 'online-only', despite them being stored locally. Here are the steps to reproduce this bug:
1. Make some mp4 file online-only using Dropbox.
2. Open AE project file which has those files used in the project.
3. AE will give you an error:

1) After Effects error: The source compression type is not supported.
2) After Effects error: file 'XXXX.mp4' cannot be imported - this 'MooV' file is damaged or unsupported.

this is understandable, since the file is online-only so it's not stored locally yet.
4. Turn off AE, and make the mp4 file available offline using Dropbox.
5. Open AE again. Despite the file being now already available on the hard drive, AE still gives you the same error and marks the file as missing. Reimporting the file again doesn't fix this.

Only ways to fix this is to either:
a) make a copy of the mp4 file and reimport it again
b) reinstall AE

I think that sometimes clearing the cache works too but its not a guarantee.

AE version 24.5 build 52
OSX Sonoma 14.5

[jenkmeister]

I believe if you clear the Conformed Media Cache (Preferences -> Media & Disk Cache), that should clear out the failure state and allow the file to be reloaded after it is made available. 

[coolguy2020]

Thanks for responding so quickly. So this means that clearing the cache with Edit -> Purge -> All memory & Disk Cache is not the same as going through Preferences -> Media & Disk Cache -> Conformed Media Cache -> Clean Database & Cache?

[jenkmeister]

Yes, the Conformed Media Cache is separate from the Memory & Disk Cache. The latter being layers and frames rendered. 

[coolguy2020]

Hey there again. Unfortunetly clearing the Conformed Media Cache isn't solving the problem. Only reinstalling AE is helping.