This is probably due to a protection policy that the NetApp uses, when a volume is at 100% full, a LUN has no guaranteed space to write new data to, so to avoid any potential corruption it offlines the LUN, which is why you are unable to connect to it.
You need to free up some space in the volume in order to mount this LUN again. Either delete some old snapshots, or grow the volume.
Well … you turned off space reservation everywhere and filled volume to the limits which means, there is nothing Data ONTAP can do to protect you from running out of space. You are solely responsible for monitoring available space and taking steps when it becomes low.
Please read TR-3483 which explains in details how space for LUN is managed on NetApp. In short, you must ensure that sum of LUN size and possible snapshot size during retention period does not exceed volume size. It does in your case.
You have to decide what is more important for you – squeezing last byte out of NetApp or ensuring continues data availability. Personally I prefer the latter ☺.
How full is file system in Windows? If there appears much free space you could try to run space reclamation on Windows, but there were some bugs resulting in data corruption, so I’d open support case to verify that you do run into it.
For now the only way is to remove more snapshots, but you probably need to increase volume size anyway.
If you are not using snapshots for the volume, you do not need to reclaim any space. If you delete the windows files, the space util will still be 96% but you can fill up the lun again if you are NOT using snapshots, no problem at all. Its just that the netapp machine itself cannot see which blocks are used or not, so once a LUN has been full, it will remain full. This is only a problem if taking snapshots, thats why you usualy need 2,3 times the space for a LUN using snapshots unless you are going for thin provisioning.
you need to delete all snapshots, then your lun will still be full but you can write 1tb to it again. also make sure that the volume is at least a little bit bigger than the lun, eg 1tb lun so have a 1,1tb volume.
if you delete and recreate, as soon as you make snapshots, the "problem" (it isnt a problem, its working as designed) will reoccur.