The error showed up when the update was complete, after the 'Applying patch' step, and the button said 'Repair'. When I did repair it, it just downloaded the entire game all over again and came up with the exact same error, with the Play-button still saying 'Repair'.
I managed to fix it though, by renaming the PTS folder to something gibberish and installing it fresh. Now however I am stuck with almost 80GB of undeletable data (I can't delete the files, but I could rename the folder).
You should verify your hard drive's integrity. There might be some problem with the actual disk. Makre sure to backup any important data as soon impossible, then try to pin down the actual issue. This might be just some minor problem with a tiny area on your hard disk, but it might as well be short before failing completely.
Since it's always annoying to search around for solutions to various errors (not just ESO, but in general) only to find threads where the OP never updated people on whether or not he found a solution and if he did, what solved it, I figured I'd post an update. I don't know what this error means, and it may not solve it for you, but...
I managed to fix it and remove the 73+GB of unusable data from my computer by:
Go to 'This PC' → Right click on 'C:\' → 'Properties' → 'Tools' → 'Check' → 'Scan drive'
Windows came up with 26 errors, and after I repaired them the issue was resolved.
Also, if anyone is getting Fatal Error 6 when they click Play in the launcher or start eso.exe:
I was getting that same error after my game crashed earlier today, and that is how I found the fix to this error too. After Windows repaired the hard drive errors I just ran a game repair (tried repairing 3 times prior to repairing with Windows and it didn't work) and it's working fine again!