Unable to write entry
Install path does not exist
Package Install: There was an error installing an extension: plg_sigplus.ar.zip
On the other Joomla 2.5.4 working correctly
If the installation of sigplus succeeds on one Joomla system but fails on another, I would suspect a problem with the operating environment (e.g. folders that should normally allow write access lack write permission) rather than a sigplus-specific issue. In any case, it is best to contact the developer directly with problems related to the extension; the Joomla review system is most unsuitable for seeking help. In particular, your e-mail is not revealed that would allow writing a response.