With this additional information, I think I understand your problem quite well. Unfortunately, Eraser installs do not seem to like downgrading, and problems can occur even if you uninstall first.
Here is a direct quote from the FAQ topic in which you 'did not find any answers':
"Sometimes, you need to uninstall Eraser (particularly if it is a 6.1/6.2 build), delete the Task list and the the registry keys (HKCU\Software\Eraser), reboot and reinstall ... Cases do arise where it is necessary to open the Registry Editor, search for all entries (keys or, where appropriate, values within a key) containing the word 'Eraser', and delete those keys or values manually."
The point is that, in my experience, once a problem of the kind you describe occurs, no amount of reinstalling will fix it. You have to uninstall, then look in both the file system and the Registry for any traces of Eraser you can find (using if you can both Registry cleaners and the search described above) and remove them, then reboot before you attempt a further install.
Do be warned : traces are easy to miss. I have on occasion had to go through the process several times before persuading Eraser to reinstall properly. So care and patience are needed here.
Why does this problem occur? To be honest, I don't know. As far as I can tell, what seems to cause the problem is not the Registry keys Eraser uses for normal running (uninstalling usually deletes these keys in any case), but keys and key values the Windows installer inserts in the system and then does not remove on uninstall. It may also be that the automated build server builds used for the 'nightly' betas are more likely to create issues. For stable builds, such as 6.0.9, Joel hand builds the install file, and that ought in principle to make installation (or, to be more accurate, uninstallation) less error-prone.