If you're getting this error:  

Oct 14 01:43:26 xcpng01 xapi: [error|xcpng01.nix.mds.xyz|3179 UNIX /var/lib/xcp/xapi|dispatch:SR.get_uuid D:3c84eaa48cb2|backtrace] SR.get_uuid D:855756122ab7 failed with exception Db_exn.DBCache_NotFound("missing row", "SR", "OpaqueRef:NULL")

Oct 14 01:43:26 xcpng01 xapi: [error|xcpng01.nix.mds.xyz|3179 UNIX /var/lib/xcp/xapi|dispatch:SR.get_uuid D:3c84eaa48cb2|backtrace] Raised Db_exn.DBCache_NotFound("missing row", "SR", "OpaqueRef:NULL")
Oct 14 01:43:26 xcpng01 xapi: [error|xcpng01.nix.mds.xyz|3179 UNIX /var/lib/xcp/xapi|dispatch:SR.get_uuid D:3c84eaa48cb2|backtrace] 1/10 xapi @ xcpng01.nix.mds.xyz Raised at file ocaml/database/db_cache_impl.ml, line 54
Oct 14 01:43:26 xcpng01 xapi: [error|xcpng01.nix.mds.xyz|3179 UNIX /var/lib/xcp/xapi|dispatch:SR.get_uuid D:3c84eaa48cb2|backtrace] 2/10 xapi @ xcpng01.nix.mds.xyz Called from file ocaml/database/db_cache_impl.ml, line 58
Oct 14 01:43:26 xcpng01 xapi: [error|xcpng01.nix.mds.xyz|3179 UNIX /var/lib/xcp/xapi|dispatch:SR.get_uuid D:3c84eaa48cb2|backtrace] 3/10 xapi @ xcpng01.nix.mds.xyz Called from file ocaml/xapi/db_actions.ml, line 13512
Oct 14 01:43:26 xcpng01 xapi: [error|xcpng01.nix.mds.xyz|3179 UNIX /var/lib/xcp/xapi|dispatch:SR.get_uuid D:3c84eaa48cb2|backtrace] 4/10 xapi @ xcpng01.nix.mds.xyz Called from file ocaml/xapi/rbac.ml, line 227
Oct 14 01:43:26 xcpng01 xapi: [error|xcpng01.nix.mds.xyz|3179 UNIX /var/lib/xcp/xapi|dispatch:SR.get_uuid D:3c84eaa48cb2|backtrace] 5/10 xapi @ xcpng01.nix.mds.xyz Called from file ocaml/xapi/rbac.ml, line 236
Oct 14 01:43:26 xcpng01 xapi: [error|xcpng01.nix.mds.xyz|3179 UNIX /var/lib/xcp/xapi|dispatch:SR.get_uuid D:3c84eaa48cb2|backtrace] 6/10 xapi @ xcpng01.nix.mds.xyz Called from file ocaml/xapi/server_helpers.ml, line 80
Oct 14 01:43:26 xcpng01 xapi: [error|xcpng01.nix.mds.xyz|3179 UNIX /var/lib/xcp/xapi|dispatch:SR.get_uuid D:3c84eaa48cb2|backtrace] 7/10 xapi @ xcpng01.nix.mds.xyz Called from file ocaml/xapi/server_helpers.ml, line 99
Oct 14 01:43:26 xcpng01 xapi: [error|xcpng01.nix.mds.xyz|3179 UNIX /var/lib/xcp/xapi|dispatch:SR.get_uuid D:3c84eaa48cb2|backtrace] 8/10 xapi @ xcpng01.nix.mds.xyz Called from file lib/xapi-stdext-pervasives/pervasiveext.ml, line 24
Oct 14 01:43:26 xcpng01 xapi: [error|xcpng01.nix.mds.xyz|3179 UNIX /var/lib/xcp/xapi|dispatch:SR.get_uuid D:3c84eaa48cb2|backtrace] 9/10 xapi @ xcpng01.nix.mds.xyz Called from file map.ml, line 135
Oct 14 01:43:26 xcpng01 xapi: [error|xcpng01.nix.mds.xyz|3179 UNIX /var/lib/xcp/xapi|dispatch:SR.get_uuid D:3c84eaa48cb2|backtrace] 10/10 xapi @ xcpng01.nix.mds.xyz Called from file sexp_conv.ml, line 148
Oct 14 01:43:26 xcpng01 xapi: [error|xcpng01.nix.mds.xyz|3179 UNIX /var/lib/xcp/xapi|dispatch:SR.get_uuid D:3c84eaa48cb2|backtrace]
Oct 14 01:43:26 xcpng01 xapi: [ info|xcpng01.nix.mds.xyz|3179 UNIX /var/lib/xcp/xapi|session.logout D:d46492e951fe|xapi] Session.destroy trackid=20d4ac64e5a5f078e0d7602753fd57ef
Oct 14 01:43:26 xcpng01 xapi: [error|xcpng01.nix.mds.xyz|3179 UNIX /var/lib/xcp/xapi||cli] Converting exception INTERNAL_ERROR: [ Cli_util.Cli_failure(“No SR specified and Pool default SR is null”) ] into a CLI response
Oct 14 01:43:26 xcpng01 xapi: [error|xcpng01.nix.mds.xyz|3179 UNIX /var/lib/xcp/xapi||backtrace] Raised Cli_util.Cli_failure("No SR specified and Pool default SR is null")
Oct 14 01:43:26 xcpng01 xapi: [error|xcpng01.nix.mds.xyz|3179 UNIX /var/lib/xcp/xapi||backtrace] 1/1 xapi @ xcpng01.nix.mds.xyz Raised at file (Thread 3179 has no backtrace table. Was with_backtraces called?, line 0
Oct 14 01:43:26 xcpng01 xapi: [error|xcpng01.nix.mds.xyz|3179 UNIX /var/lib/xcp/xapi||backtrace]

It means you don't have a default SR defined in XCP-ng Center 8.0.1 .  Simply visit XCP-ng Center, right click on one of the SR's and make it a default.  

Thx,
TK