I don't mean to revive old threads, but I had the exact same problem with my forum yesterday. "Session verification failed. Please try logging out and back in again, and then try again." and there were no entries in the error log. I just wanted to point out the circumstances under which SMF threw this error.
I was messing around with
index.php,
proxy.php... basically, all of the script files in the root. I was trying to make the line endings Windows compatible (CR + LF) and also saved them as UTF-8 instead of ANSI. I didn't thought SMF would mind this (most servers recognize UTF-8 and CR + LF line end/new line), but I was wrong. After I saved them and copied them via FTP (binary mode, not ASCII), while I was logged in, I logged out and couldn't log in afterwards. SMF threw the previously mentioned error.
I just downloaded a fresh copy of the adequate SMF version I had on my forum (in my case 2.0.15) copied all of the files I altered and voila

, everything was working fine

.
I was just wondering if someone could give an explanation why this happened

. I thought the script formatting wouldn't matter (most servers are Debian based, CR + LF and UTF-8 in text files was implemented a while ago), so I really couldn't find an explanation to why this happened. Maybe this particular server threw this error (I'm on 1and1, I know, I have to switch to another hosting service

)

. Maybe the fact that I was logged in and logged out after I altered the files was the trigger to this behavior? Maybe the script makes a binary check (CRC) of some it's files during log in and out just to make sure nobody tempered with them?
Any insight on the issue would be appreciated

.
Just to be clear, I've solved the problem so I don't need help solving it, I'm just curious

.