Why won't my Lightroom 5.3 import files anymore?

Question

I'm on a Mac with Lightroom 5.3 freshly installed. It worked neatly for a couple of days when importing either from my iPhone or my 6D (from the card reader). Suddenly it stopped doing that. I've been through all the discussions and I've tried the following steps:
- delete the preference file (and re-start Lightroom)
- re-install Lightroom
- create a new catalogue (won't work either)
- checked the permissions (as in the beginning, it would tell me that it can't read my files - but they worked perfectly!), which were fine
- cleared the "recently used" file paths of the import dialogue
- re-booted my Mac about a gazillion times
- updated the RAW software
- re-installed Lightroom again (but having deleted pretty much everything from the HD that was Lightroom related)
It just won't work. But before I re-installed Lightroom, it would still give me the "error: can't read files" sign. Now, it simply starts importing (the progress bar starts anyway) and then it just stops at one point, not having imported anything. Lightroom doesn't freeze, it just doesn't do anything and the progress bar stops. I wanted to  attach a screenshot to show you what it looks like right now, but that didn't work either, so whatever.
Here's the thing: it still works in Lightroom 4. So now I've been importing into Lightroom 4 and adding the files without moving them to Lightroom 5. But what kind of workflow is that? It's literally killing me because I've paid money for the software and I was patient enough to go over all the trouble shooting, but I'm really out of wit. I hope somebody can help me with this problem.
Thanks in advance.

Answer 1

This is my observation:
My Pictures and my Movies folder are on an external harddisk. I have an alias for both of them in my User folder.
The Finder can access the catalogs on the external harddisk without problems.
When I create a catalog on my SSD, detach the external drive, delete the aliases and create folders Pictures and Movies on the SSD everything is fine.
When I delete the folders Pictures and Movies on the SSD again, attach the external drive and re-create the aliases things go wrong.
The problem is that Lightroom can't handle the alias! That's all.
This problem has never been an issue until the lastest version (5.7) of LR.
Whatever I do in terms of access rights to the alias and/or the folder the alias points to, it won't work.
Please fix this URGENTLY!
Reiner
MacBook Pro
Mac OS X 10.10
Lightroom 5.7