@markus Welcome to the Rockstor community.
Which version of Rockstor are you using to attempt this poorly pool mount? I would strongly suggest that you try our non legacy Rockstor 4 which is at Release Candidate 7 stage (once updated to the latest available within the testing channel):
There is as yet no installer download available but you can make your own with all updates auto pre-applied via the instructions available in the following GitHub repo:
Best if you use a seperate system disk for this install and don’t have the original Rockstor 3 system disk attached. It confuses the new install. That way if you do get the pool back you can always revert to the prior system disk and pick-up where you left off. Although a re-config of the Rockstor 4 to your requriements would be better but that is not the question here.
That way at least you get a far more modern btrfs subsystem than you might otherwise be using if you are still using our now legacy CentOS variant of Rockstor 3.
The huge number of fixes in the stack between these versions may well increase your changes of establishing a successful mount. Your pool may also be sufficiently poorly to require not only a degraded option but also a ‘ro’ read only option. This would at least enable you to refresh your back-ups.
From your log it does look like sda also has errors:
A good practice in this situation is to double check your memory health also. as both the removed device bdev (null) had errors of read and write as well as the still remaining sda. And raid5 can only handle a single drive failure, where as these stats tend to indicate 2 drives have gone fault. Hence the memory check just in case. See our: Pre-Install Best Practice (PBP):
http://rockstor.com/docs/pre-install-howto/pre-install-howto.html#pre-install
specifically the Memory Test (memtest86+) section:
http://rockstor.com/docs/pre-install-howto/pre-install-howto.html#memory-test-memtest86
and double check, re-connect all cables.
Hope that helps.