I believe that the plugin Thread Form is conflicting with JCoins to some level. If I insert text in the Shop item description and leave BBCode option unchecked, the ACP crashes returning a fatal error. Looking up the error code I noticed this warning:
An update of package de.wcflabs.wbb.threadForm from version 1.1.0 to 1.1.2 is not supported.
I think that there might be some code clashing somewhere between the JCoins and Thread Form plugins.
@Josh The issue persists even after uninstalling Thread Form plugin. Once both plugin are installed it seems they clash, and removing thread form still won't fix the problem.