Rootkit - Avast Error code 05

[REDACTED]

I have set a full scan on my computer a couple of days ago. MalwareBytes found nothing, but Avast! found a rootkit file, which it treated like multiple threats (precisely 6987), and when I try to fix it automatically, repair it, move it to Quarantine or delete it, I get an error. Most times it's Error 5, but I get Error 50 too. Turns out the path for the file it found "harmful" doesn't even exist, or maybe it used to, but now it i gone. Just to be sure, I scanned the computer again with MalwareBytes and the folder where the file was "infected" with Avast! itself, and found nothing.
Is it a false positive or should I worry about it?

Thanks in advance for the response 

[Lisandro]

I've addressed your question in the PT forum: https://forum.avast.com/index.php?topic=200090
Here is the automated translation of it:

Welcome to the forums!

"Fading" files or folders can be a sign of rootkits. This is exactly their behavior.
Error 5 is due to access blocking by Windows (or by the malware driver / service itself).
I suggest a full scan on reboot (Protection> Antivirus> Other scans> Scanning on reboot).

On the other hand, yes, it is possible to be a false positive. Avast generates a scan report / history. There should be path information, file name detected and malware.