Jira upgrade from 5.2.11 to 6.3.12

[Govardan Krishna]

I am trying to upgrade JIRA from 5.2.11 to 6.3.12 but i am having issues:

Exception thrown during upgrade: com.atlassian.util.concurrent.LazyReference$InitializationException: com.atlassian.util.concurrent.LazyReference$InitializationException: org.springframework.beans.factory.BeanCreationException: Error creating bean with name 'com.keplerrominfo.jira.plugins.usergrouppicker.UserCFType': Instantiation of bean failed; nested exception is org.springframework.beans.BeanInstantiationException: Could not instantiate bean class [com.keplerrominfo.jira.plugins.usergrouppicker.UserCFType]: Constructor threw exception; nested exception is java.lang.NoSuchMethodError: com.atlassian.jira.issue.customfields.impl.UserCFType.(Lcom/atlassian/jira/issue/customfields/persistence/CustomFieldValuePersister;Lcom/atlassian/jira/issue/customfields/converters/UserConverter;Lcom/atlassian/jira/issue/customfields/manager/GenericConfigManager;Lcom/atlassian/jira/config/properties/ApplicationProperties;Lcom/atlassian/jira/security/JiraAuthenticationContext;Lcom/atlassian/jira/bc/user/search/UserPickerSearchService;Lcom/atlassian/jira/issue/fields/rest/json/beans/JiraBaseUrls;Lcom/atlassian/jira/user/UserHistoryManager;)V com.atlassian.cache.CacheException: com.atlassian.util.concurrent.LazyReference$InitializationException: com.atlassian.util.concurrent.LazyReference$InitializationException: org.springframework.beans.factory.BeanCreationException: Error creating bean with name 'com.keplerrominfo.jira.plugins.usergrouppicker.UserCFType': Instantiation of bean failed; nested exception is org.springframework.beans.BeanInstantiationException: Could not instantiate bean class [com.keplerrominfo.jira.plugins.usergrouppicker.UserCFType]: Constructor threw exception; nested exception is java.lang.NoSuchMethodError: com.atlassian.jira.issue.customfields.impl.UserCFType.

[Boris Berenberg]

This is probably because the Kepler plugin is out of date. Try removing all the files from the <JIRA HOME>/plugins/installed-plugins directory then restarting JIRA. If the issue persists, I recommend filing a support ticket with us at: https://support.atlassian.com/

[Govardan Krishna]

Thank you Boris, i have removed all files from jira_data/plugins/installed-plugins directory and restarted JIRA worked.

After i login to JIRA/Add-ons, i got below error

The Atlassian Marketplace server is not reachable. To avoid problems when loading this page, you can disable the connection to the Marketplace server. Click here for more information...

If i disable connection, how do i install plugins?

Your help is appreciated