Add to cart not working after 2.6.10 -> 2.9.9.e upgrade

[SteP[IT]]

I have cloned one of my websites for testing purpose, and upgraded it from 2.0.26d to the latest 2.6.10: after this upgrade, all works normally, and the shop is fully functional.
Then I have tried to upgrade it again, form 2.6.10 to 2.9.9e: after this upgrade the "Add to cat" button doesn't add any item to the cart anymore.
Any hint about? The global configurations has not been changed during the upgrade, so I can't recognize where to look at...
The test configuration is:
- Joomla 2.5.27
- Apache 2.4.10
- Php 5.5.16
Thank you in advance for  any help
Best Regards

[pigletdcat]

Same here...until I found out is that Version 2.9.9 has many problems and unstable. I would have to start over again.

[SteP[IT]]

version 2.9.9.3 has fixed that, but is still sligthly bugged...
The VM team is doing a great job, following our bug reports and working hard to fix everything: I presume they wiil shortly release a stable enough version