Json error after upgrade from 2.5.28 to 3.6.5 - Joomla! Forum - community, help and support

hello

after upgrade joomla version 2.5.28 3.6.5 gain errors.
please see 1 below json encoding error in specifics k2 categories swmaps plugin include below:

oops... error decoding json data: syntax error
error decoding json data: syntax error

call stack
# function location
1 japplicationcms->execute() /home/ferramew/public_html/36/index.php:49
2 japplicationsite->doexecute() /home/ferramew/public_html/36/libraries/cms/application/cms.php:261
3 japplicationsite->dispatch() /home/ferramew/public_html/36/libraries/cms/application/site.php:230
4 jcomponenthelper::rendercomponent() /home/ferramew/public_html/36/libraries/cms/application/site.php:191
5 jcomponenthelper::executecomponent() /home/ferramew/public_html/36/libraries/cms/component/helper.php:380
6 require_once() /home/ferramew/public_html/36/libraries/cms/component/helper.php:405
7 jcontrollerlegacy->execute() /home/ferramew/public_html/36/components/com_k2/k2.php:63
8 k2controlleritemlist->display() /home/ferramew/public_html/36/libraries/legacy/controller/legacy.php:702
9 k2controller->display() /home/ferramew/public_html/36/components/com_k2/controllers/itemlist.php:49
10 jcontrollerlegacy->display() /home/ferramew/public_html/36/components/com_k2/controllers/controller.php:20
11 k2viewitemlist->display() /home/ferramew/public_html/36/libraries/legacy/controller/legacy.php:664
12 jeventdispatcher->trigger() /home/ferramew/public_html/36/components/com_k2/views/itemlist/view.html.php:158
13 jevent->update() /home/ferramew/public_html/36/libraries/joomla/event/dispatcher.php:159
14 call_user_func_array() /home/ferramew/public_html/36/libraries/joomla/event/event.php:69
15 plgk2swmap->onk2categorydisplay()
16 joomla\registry\registry->__construct() /home/ferramew/public_html/36/plugins/k2/swmap/swmap.php:1403
17 joomla\registry\registry->loadstring() /home/ferramew/public_html/36/libraries/vendor/joomla/registry/src/registry.php:72
18 joomla\registry\format\json->stringtoobject() /home/ferramew/public_html/36/libraries/vendor/joomla/registry/src/registry.php:366

----------------------------------------end error--------------------------------------------------------

my server site ground running php version 5.6.30

any please

regards

there have been reported issues json decode error associated k2.

you can check topics such http://www.joomlaworks.net/forum/k2-en/46914-0-error-decoding-json-data-syntax-error , https://www.concertodesigns.com/web-design-solutions/item/58-fix-json-syntax-error-with-imported-k2-content. though these topics may not have same issue having, may serve guidelines in tracing issue.

what suggest create new set of test data k2 category, k2 article, , forth related issue having. analyze data format stored ones having issues. find pattern. once pattern known easier fix issue.