The traffic-light custom field is derived from the native option field of JIRA. Unfortunately, Atlassian seems to have changed the format with regard to JIRA 7.
Assuming, that you have access to your JIRA database as administrator/dba, please look in the genericconfiguration table:
the default value for the custom field that is broken has got an entry like <string>####</string> and it should have got the new entry like <long>####</long> in the xmlvalue column.
Please change the tag in the old entry from <string> to <long> in order to fix that problem (no more 500 errors and everything is displaying ok).
Use the following SQL-statement to detect that records: