There isn't enough data provided here to troubleshoot for certain, but I am pretty confident that you have a misbehaving device driver loaded on your system(s) that needs updating.
fltmgr.sys is the Filter Manager kernel mode driver. Many so-called "mini-filters" plug in to this Filter Manager architecture. These mini-filters are provided both by Microsoft and by third-party vendors to provide additional functionality to the system, such as on-access antivirus scanning, file and folder redirection, etc. So the bug check is pointing to fltmgr.sys, but fltmgr.sys is just an "umbrella" that's covering the underlying bad actor.
The APC_INDEX_MISMATCH stop code means that a driver called a routine to disable an APC (asynchronous procedure call) without also calling its matching routine to re-enable the APC when it left a guarded or critical region.
Developers have to be very careful when writing device drivers and it's easy to make mistakes when writing device drivers.
You'd use a kernel debugger with the crash dump to further investigate exactly which thread was the culprit, which would lead you to a particular device driver, which I would bet money needs to be updated.
"Update your drivers" is about all I think we can give you with the data that we have.