From the changelog:

* Forgot o2 germany sources in the last build!!
* Removed post-uninstall script. If you can't uninstall 0.1.1-7, please do "rm /var/lib/dpkg/info/web2sms.postrm"

This means: Go to X-Terminal, become root, insert the "rm /var/lib/dpkg/info/web2sms.postrm" command and then uninstall web2sms with "apt-get remove --purge web2sms" or with the Application Manager.
After that you can install the current version from extras-devel without a problem

And please don't write ten times thanks for that if it was helpful, just press the "Thanks" button