When I purge cache and find updates, I keep getting this error message:

Update: :Extension: Could not open /extension.xml

This only happened after I had uninstalled SIG and installed SIG 3.0.

On another site, I get this error after clearing cache and finding updates:

Update: :Extension: Could not open project.joomexp.com/nhk/testupdate/extension.xml

The SIG 3.0 version shows as needing to be updated BUT it has already been updated.

If I go ahead and update, clear cache and find updates again, I get the exact same error message and the SIG shows an update for 3.0 that needs to be installed.

What Joomla version do you use and which SIG version are you trying to install?

What Joomla version do you use and which SIG version are you trying to install?

JoomlaWorks 'Simple Image Gallery' Plugin v3.0.0 for Joomla!
Joomla v2.5.14.

Are you sure you are trying to install  SIG Pro v.3.0 for Joomla 2.5?

Not the pro version, the free version.

I meant SIG  sorry.
Please install Admin Tools from www.akeebabackup.com/ in order to fix your permissions as they should be and then reinstall the extension.

Right permissions on all joomla sites:
755 for folders
644 for files