I mean that I see two versions installed in the module loader. The version 2.5.2. didn't disappear after installing later versions. So it looks like 2.5.2 is still installed next to the actual version.
I'm facing the same issue, but there is no SecurityGroups.php in the custom/Extension/modules/Administration/Ext/Administration folder. There's only one file called securitygroupsadminoption.php.
Working on CE 6.5.16 and Security Groups 2.6.8. But also 2.5.2 is still there.