This is usually the result of a bad cache. Try this.
1. Open a new, empty PP project.
2. Go to Preferences>Media... and take note of where your Media Cache files are stored.
3. Below that, Clean the Media Cache Database.
4. Close PP and use Explorer or Finder to delete the Cache files you located in step 2.
5. Reopen your project, and wait for new cache files to be built.
Thanks Jim, but it didn't work for me, the files didn't reconnect nor did Pr prompt me that I had offline files when I opened the project.
I believe the issue stems from the newer REDimporter being incompatible with projects that were created using an older version.
Once I replaced the REDimporter, I still suffered with issues 1 and 2 in my original post, but I was able to manually reconnect without the error.
It happened to me as well, several times.
It is a REDimporter problem.
Once I solved the problem by going back to an older version of the REDimporter, but that happened on a CS5.5 version. CS6 seems to have several problems with R3D media, and I can only wish you good luck.
It is a random issue and it seems to appear and disappear without a real reason.