Adobe Media Encoder error - the file could not be imported. Could not open source file.

Try this:

https://www.autokroma.com/blog/BETA-For-Influx-Importer-Adobe-Premiere-Pro-After-Effects

Suffering from this for a year when rendering After Effects comps in AME. The problem is that the temp files that AME creates don't have the correct permissions. My solution is, when the Ae comps are in the AME queue, to go to the folder, change the permissions to Read & Write for everyone, and then start the encoding queue.

Quite a hazzle - hope this ever ends.

I am on MacOS Catalina.

YAY! I HAVE THE ANSWER!!!!  So...I did a chat with an Adobe Cloud agent; I already had Handbrake installed so I was lucky otherwise I would have been apprehensive about this solution (I use Handbrake to fix synch problems with music videos).  If you're not familiar, Handbrake is a program I downloaded free from the internet. I HATE doing that but again, I *really* needed a free program to synch my music videos. I'm not made of $$$ haha...anyway, I simply dumped the video into Handbrake, clicked the Video tab, there was an option to change the frame rate to "constant frame rate." I did that, rendered it in Handbrake and, VOILA! It converted my clip into a VIABLE .mp4.  I AM SOOOOOO RELIEVED. I frequently do special effects and render first in iMovie, but it renders the clip into a .mov file.  Anyway, hopefully this will help some of you and this isn't too late for your projects - at least it will help you all (and hopefully others) with your projects going forward.  Good luck everyone! 

Likely being caused by one or two issues. 
First, temp files are probably being created on the tntdrive by Adobe.  Due to tntdrive using a background queue/cache, the deletion of the semaphore temp files may not be immediate.  Maybe change the cache options to resolve that.  

second possibility, S3 allows for case sensitive file and path names.  Therefore it is possible that path references may not find the files they need.  Try putting all of your files in lower case letters.  

I have the same issue. I've looked around and I can't see that Adobe's acknowledged the problem or offered a solution.

Changing the file from .mov to .mp4 solved the issue for me. But not getting any answer from the company is scary - and unprofessional.

I'm having the same problem.

Hello #Adobe, what do I do now?

I have the same problem that just appeared this week. I had no problems last week importing the same file types. The error appears in both Premiere Pro and Media Encoder for four like files. Media Encoder supplies the most detailed error message. "mov file could not be imported. Could not open source file. Please check that the file exists with correct permissions."