I do most of my NSO development in erlang, and run my development instances in the foreground, where stack traces of these internal errors can be seen. And then experience tells me that when data is displayed in the CLI like "--ERROR--" it usually means a stack trace is produced.
You can also see historic stack traces with the "ncs --printlog" eg:
kri@desk:~/ncs-nbp/logs$ ncs --printlog ncserr.log.1
Eshell V10.7.2.9 (abort with ^G)
{"12-Feb-2021::12:26:18.096",
{error_report,<0.1853.0>,
{<0.1857.0>,crash_report,
[[{initial_call,{cli_backend_server,'-start_link/4-fun-0-',[]}},
{pid,<0.1857.0>},
{registered_name,[]},
{error_info,
{error,
{badmatch,<<>>},
[{cli_backend_server,read_init2,5,
[{file,"cli_backend_server.erl"},{line,1129}]},
{cli_backend_server,init,5,
[{file,"cli_backend_server.erl"},{line,1002}]},
{proc_lib,init_p,3,[{file,"proc_lib.erl"},{line,234}]}]}},
{ancestors,[<0.1853.0>,cli_server,cli_sup,<0.522.0>]},
{message_queue_len,1},
{messages,[terminal_input_ping]},
{links,[<0.1853.0>]},
{dictionary,[]},
{trap_exit,false},
{status,running},
{heap_size,376},
{stack_size,27},
{reductions,364}],
[]]}}}
1> 12-Feb-2021::12:29:30.067 <0.1866.0> <0.1998.0> rollback:814: cs_trans_rng:show_delta_rev(): throw:{error,{cs_error,undefined,bad_value,bad_value,undefined,undefined,undefined,{[{{13,666}},8235672,{<<"ANON">>,<<"ANON">>,{10,1}},760089515,[702437083|205751944],1787219742,[734664653|962606743]]},undefined,false}}
[{cs,val2str,7,[{file,"cs.erl"},{line,1856}]},
{cs_trans_rng,qval2str,7,[{file,"cs_trans_rng.erl"},{line,5714}]},
{cs_trans_rng,emit_leaf,7,[{file,"cs_trans_rng.erl"},{line,7870}]},
{cs_trans_rng,get_leaf,7,[{file,"cs_trans_rng.erl"},{line,3573}]},
{lists,foldl,3,[{file,"lists.erl"},{line,1263}]},
{cs_trans_rng,get_cs_delta1,8,[{file,"cs_trans_rng.erl"},{line,6763}]},
{cs_trans_rng,'-show_delta_rev/14-fun-4-',19,
[{file,"cs_trans_rng.erl"},{line,6187}]},
{cs_trans_diff_iterate,sh_diff_iterate_x,10,
[{file,"cs_trans_diff_iterate.erl"},{line,323}]},
{xds_ram,tts_fold_children,7,[{file,"xds_ram.erl"},{line,1259}]},
{cs_trans_diff_iterate,sh_diff_iterate_x,10,
[{file,"cs_trans_diff_iterate.erl"},{line,491}]},
{xds_ram,tts_fold_children,7,[{file,"xds_ram.erl"},{line,1259}]},
{cs_trans_diff_iterate,sh_diff_iterate_x,10,
[{file,"cs_trans_diff_iterate.erl"},{line,499}]},
{xds_ram,tts_fold_children,7,[{file,"xds_ram.erl"},{line,1259}]},
{cs_trans_diff_iterate,sh_diff_iterate_x,10,
[{file,"cs_trans_diff_iterate.erl"},{line,499}]},
{cs_trans_diff_iterate,sh_diff_iterate,7,
[{file,"cs_trans_diff_iterate.erl"},{line,131}]},
{cs_trans_rng,'-show_delta_rev/14-fun-11-',3,
[{file,"cs_trans_rng.erl"},{line,6360}]},
{cs_trans_rng,show_delta_rev,14,[{file,"cs_trans_rng.erl"},{line,6358}]},
{rollback,create_pending2,7,[{file,"rollback.erl"},{line,776}]},
{rollback,create_pending,7,[{file,"rollback.erl"},{line,656}]},
{cs_trans,do_rollback,8,[{file,"cs_trans.erl"},{line,17361}]},
{cs_trans,sh_validate2,13,[{file,"cs_trans.erl"},{line,12451}]},
{cs_trans,sh_validate0,7,[{file,"cs_trans.erl"},{line,12321}]},
{cs_trans,handle_call,3,[{file,"cs_trans.erl"},{line,4714}]},
{gen_server,try_handle_call,4,[{file,"gen_server.erl"},{line,677}]}]
1>
From this trace example you can see that the rollback feature is trying to convert the internal Erlang representation to a string, but likely due to some yang models being removed it does not know the string representation for all the numeric tag values.
And it all sort of makes sense, if you have a look at some of these rollback files, since they need to contain the reverse of your delete action, and if you can't "see" what data you currently have it is hard to generate the reverse.
