* 64-bit Linux version or basically any flavor of linux that doesn't have binary compatibility with 32 bit Ubuntu 12.04 LTS.
* Windows 8 where you haven't used one of the workarounds mentioned on other places in the forum
The only other cause is the flakiness of Chrome's NPAPI implementation. Sometimes when your browser starts or you initially install the plugin, the native component fails to start. The browse button depends on this part to work it's magic. Disable and re-enabling the extension should eventually solve the problem.