Ubuntu 14.04.4 LTS
mysql Ver 14.14 Distrib 5.5.47

what's shown in the lld rule list ?
how do you determine it's "hung in a disabled state" ?

Actually, disabled is possibly the wrong word. The Status is "Enabled", however there's a red cross in the Info column, and when you hover over it it gives the error "Value should be a JSON object."
I can see Zabbix executing the rule in the logs, as above, but they all end with that "End of lld_rows_get():FAIL" & no items or triggers are added from the prototypes (the configurations of which are in the screenshots above).

Log after LLD rule deleted & recreated. The invalid JSON warning no longer appears in the Info column, however the items still aren't being added & the log still ends with:

In lld_rows_get()
End of lld_rows_get():FAIL

it looks like a support case to me - no indication of a bug. i'd suggest discussing it in the available support channels (IRC etc) : http://zabbix.org/wiki/Getting_help

I spent a couple of hours with the guys on IRC, and it was finally decided a bug should be raised.

for the record, we continued this a bit more on IRC, some further testing could provide more info

The invalid JSON warning no longer appears in the Info column, however the items still aren't being added...

Is there any error message in the "Info" column now or is it green?

no matter how this particular case turns out, the following is a quite terse debug output :

7849:20160324:143255.502 In lld_rows_get()
7849:20160324:143255.502 End of lld_rows_get():FAIL

can we output something more useful here maybe ?

So, since I wrote that the warning disappeared after I deleted & re-created the rule, it's now come back.
Status: Enabled (green), Info: "Value should be a JSON object."

I created a separate test rule today, and the items were added as expected.

*** Discovery rule ***
Name: Test LLD
Type: Zabbix agent
Key: test.lld
zabbix_agentd.conf: UserParameter=test.lld[*],/mnt/pool0/zabbix/testlld.sh $1

------ testlld.sh -------
#!/bin/bash
if [[ -z $1 ]]; then
        echo '{"data":[{"{#MACTEST}":"item1","{#MACTEST2}":"abc"},{"{#MACTEST}":"item1","{#MACTEST2}":"def"},{"{#MACTEST}":"item2","{#MACTEST2}":"abc"},{"{#MACTEST}":"item3","{#MACTEST2}":"abc"},{"{#MACTEST}":"item4","{#MACTEST2}":"abc"},{"{#MACTEST}":"item4","{#MACTEST2}":"def"},{"{#MACTEST}":"item5","{#MACTEST2}":"abc"},{"{#MACTEST}":"item6","{#MACTEST2}":"abc"},{"{#MACTEST}":"item6","{#MACTEST2}":"def"},{"{#MACTEST}":"item7","{#MACTEST2}":"abc"},{"{#MACTEST}":"item8","{#MACTEST2}":"abc"},{"{#MACTEST}":"item9","{#MACTEST2}":"def"},{"{#MACTEST}":"item10","{#MACTEST2}":"abc"},{"{#MACTEST}":"item11","{#MACTEST2}":"def"},{"{#MACTEST}":"item12","{#MACTEST2}":"abc"},{"{#MACTEST}":"item13","{#MACTEST2}":"abc"},{"{#MACTEST}":"item14","{#MACTEST2}":"abc"},{"{#MACTEST}":"item15","{#MACTEST2}":"abc"}]}'
else
        echo "$1"
fi
----------------------

*** Item prototype ***
Name: Macro Test: {#MACTEST} {#MACTEST2}
Type: Zabbix agent
Key: test.lld[{#MACTEST},{#MACTEST2}]
Type of information: Character

Going back to my original rule & JSON, I changed the discovery rule to "Zabbix trapper" & used zabbix_sender to send in the JSON (as returned by zabbix_get) to the Zabbix server. The moment I did that, all of the items appeared & started generating data as expected (seen in "Latest data").

root@dingham:~# zabbix_sender -z monitor0 -s "backup0" -k backup.descobrir -o '{"data":[{"{#DSET}":"mdl_InglesInfoEdu","{#DSRV}":"mysql"},{"{#DSET}":"Evento2016","{#DSRV}":"mysql"},{"{#DSET}":"serve-NP","{#DSRV}":"mysql"},{"{#DSET}":"mdl_japao","{#DSRV}":"mysql"},{"{#DSET}":"GDE_PEDA2012","{#DSRV}":"mysql"},{"{#DSET}":"srv0","{#DSRV}":"mysql"},{"{#DSET}":"sites0","{#DSRV}":"mysql"},{"{#DSET}":"mdl_Ped0709","{#DSRV}":"mysql"},{"{#DSET}":"ESP_EduCamp","{#DSRV}":"mysql"},{"{#DSET}":"srv2","{#DSRV}":"mysql"},{"{#DSET}":"inter0","{#DSRV}":"mysql"},{"{#DSET}":"srv-finalizados","{#DSRV}":"mysql"},{"{#DSET}":"mdl_PMD","{#DSRV}":"mysql"},{"{#DSET}":"mdl_AdmPub","{#DSRV}":"mysql"},{"{#DSET}":"sites1","{#DSRV}":"mysql"},{"{#DSET}":"srv1","{#DSRV}":"mysql"},{"{#DSET}":"Srv-GCDH","{#DSRV}":"mysql"},{"{#DSET}":"mdl_Eja","{#DSRV}":"mysql"}]}'
info from server: "processed: 1; failed: 0; total: 1; seconds spent: 0.134209"
sent: 1; skipped: 0; total: 1

I changed the rule type back to "Zabbix agent" & the warning in the Info column has come back, "Value should be a JSON object".

that looks very much like something is wrong with the json indeed. try :

• set it back to trapper, send in value that makes it work
• bump loglevel for trappers to 4, tcpdump zabbix port on the agent (to have less extra traffic - i doubt network would mangle it)
• change it back to agent

it is likely to fail again. carefully check debug output & tcpdump output. does the json still look ok ?

OK, all resolved, and it turns out to be a "PEBKAC" issue.

I came across this very similar issue raised in 2014 & found the key in the comments:
https://support.zabbix.com/browse/ZBX-7854?focusedCommentId=101692&page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel#comment-101692

In short, instead of adding an extra condition to the loop in my script & dropping the trailing comma there, I cheated & closed the JSON prefixed with a backslash escaped backspace & sent the output through echo.
This all appeared fine when receiving the output to the shell (using zabbix_get & executing the script manually), and it also didn't show anything unusual when parsing the logs with grep & less. But opening the log in vi, it was clear that the unescaped ",\b" was being included in the stream.

/me goes back to scriptkiddie school.