Thank you to everybody for your suggestions.
Deleting the browser's cache didn't work (I am using Firefox18).
ONTAP and OnCommand SM versions are compatible.
All snapshots were deleted AFAIK (There are clear error messages telling one which snapshots have to be deleted for downgrading to proceed).
I removed the system from OnCommand SM and... can't re-discover it anymore.
I guess, ssl/ssh certificates are relevant for the login but not for the discovery.
Sure I've exited and logged in into OnCommand SM.
What else could one try?
There is on interesting moment. Namely, the controller itself ("snap list" command) reports that there are only post-downgrading (later than March 3) snapshots, yet the admin host finding pre-downgrading directories and files.
On the controller:
gen-netapp2> snap list vol_10gb
Volume vol_10gb
working...
%/used %/total date name
---------- ---------- ------------ --------
0% ( 0%) 0% ( 0%) Mar 07 12:54 gen-netapp5(0118050929)_vol_10gb_m.2203 (snapmirror)
0% ( 0%) 0% ( 0%) Mar 07 12:00 hourly.0
0% ( 0%) 0% ( 0%) Mar 07 08:00 hourly.1
0% ( 0%) 0% ( 0%) Mar 07 00:00 nightly.0
0% ( 0%) 0% ( 0%) Mar 06 20:00 hourly.2
0% ( 0%) 0% ( 0%) Mar 06 16:00 hourly.3
0% ( 0%) 0% ( 0%) Mar 06 12:00 hourly.4
0% ( 0%) 0% ( 0%) Mar 06 08:00 hourly.5
0% ( 0%) 0% ( 0%) Mar 06 00:00 nightly.1
On the admin host:
[root@vh13 .snapshot]# pwd
/netapp2/vol_10gb/.snapshot
[root@vh13 .snapshot]# ls -l
total 36
drwxr-xr-x 5 root root 4096 Mar 6 15:19 gen-netapp5(0118050929)_vol_10gb_m.2204
drwxr-xr-x 5 root root 4096 Mar 6 15:19 hourly.0
drwxr-xr-x 5 root root 4096 Mar 6 15:19 hourly.1
drwxr-xr-x 5 root root 4096 Mar 6 15:19 hourly.2
drwxr-xr-x 5 root root 4096 Mar 6 15:19 hourly.3
drwxr-xr-x 5 root root 4096 Mar 6 11:42 hourly.4
drwxr-xr-x 5 root root 4096 Feb 26 10:32 hourly.5
drwxr-xr-x 5 root root 4096 Mar 6 15:19 nightly.0
drwxr-xr-x 5 root root 4096 Feb 26 10:32 nightly.1
[root@vh13 .snapshot]#
[root@vh13 .snapshot]# ls -l hourly.5
total 501136
drwxrwxrwx 2 root root 4096 Feb 27 07:27 qt_10gb
drwxrwxrwx 2 root root 4096 Feb 27 07:31 qt_10gb_2
drwxrwxrwx 2 root root 4096 Feb 28 07:57 qt_10gb_3
-rw-r--r-- 1 root root 102400 Feb 26 10:56 urandom100kb
-rw-r--r-- 1 root root 32768 Feb 26 10:32 urandom32kb
-rwxr-xr-x 1 root root 512000000 Feb 26 11:36 urandom500mb