It happens almost every time I start a new instance--from a separate copy of the Iron Portable folder, of course. The non-installable version from srware.com does not suffer from this problem (I prefer the portableapps.com one since I hope to be able to upgrade the browser without losing extensions and customizations, by running the newest paf installer). In any case, that means it has something to do with the PortableApps packaging, I'd say. Running multiple totally separate instances is the main reason I am not using installed browsers. My OS is Win7 x64 SP1 and this applies to the latest Iron version 23.0.1300.0 (170000).
Here is what Windows says after the above crash occurs:
Problem Event Name: APPCRASH
Application Name: Iron.exe
Application Version: 23.0.1271.64
Application Timestamp: 50a8cef0
Fault Module Name: iron.dll
Fault Module Version: 23.0.1271.64
Fault Module Timestamp: 50a8ce79
Exception Code: c0000005
Exception Offset: 01fa6c78
OS Version: 6.1.7601.2.1.0.256.4
Locale ID: 1033
Additional Information 1: 0a9e
Additional Information 2: 0a9e372d3b4ad19135b953a78882e789
Additional Information 3: 0a9e
Additional Information 4: 0a9e372d3b4ad19135b953a78882e789
The crash does not happen if I put a copy of the (alas, no longer supported) adblock.ini file from srware.net/downloads/adblock.ini in the App\Iron folder in each of my portable Iron copies.
Then I realized that I had those adblock.ini files present when I was testing with the portable Iron from srware, and I went back and tested without them. Sure enough, the original srware version also crashes, even as a single instance, unlike the portableapps.com one. I put empty adblock.ini files in there and it stopped crashing. In fact, the srware version comes by default with an empty adblock.ini file, so the browser must expect that file to be present.
In contrast, the portableapps version had the adblock.ini file stripped away, and that seems to cause the crash, but only with multiple instances. Putting an empty or non-empty adblock.ini file in each App\Iron folder stopped the multiple instances from crashing, so that is the likely fix.
Apparently what I described above is only part of the story. After a few hours, the crashes started coming back at times when I'd start a separate new instance of Iron Portable. Now the issue is related to the IronPortable.exe process. Launching a\IronPortable.exe starts up that process and a few a\App\Iron\iron.exe ones. If a\IronPortable.exe is still running when I try to start b\IronPortable.exe, the latter may crash. If however, I manually kill a\IronPortable.exe first (which seems to have no influence on the running a browser other than a complaint about cleaning up on restart), then I can start the b browser just fine.
As it happened again, I got a chance to check the Windows signature and it is exactly the same as the previous one (see above). So it just seems that the problem is very intermittent and may not depend on the adblock.ini file at all in the end.
The workaround with killing the existing IronPortable.exe process continues to work, so it appears to be for real.
I finally got tired of applying the above fix and switched to the original portable version from the srware site. It may not be as "well packaged" as this one, but it has no problems with multiple instances from separate copies. Sorry.
The copy of Iron included in Iron Portable is unmodified. It does use the --user-data and --lang command line switches to get it to point to a specific directory and then move bits back and forth so Iron can find them.
Sometimes, the impossible can become possible, if you're awesome!