I'm not sure if the issue is what Demerit is suggesting (join limits), or one of database length limits. Your CiviCRM log should give a more detailed error. If you find the error, you can edit your question to include it for better help.
I suspect you're hitting a database length limit. The solution depends on your version of MySQL/MariaDB, but I'm ...
You could create a custom field with the same name that you want for the option_group, then the option_group would have your friendly name and you can select this for your existing and new fields. You don't do anything else with the special custom field and you could hide it in a suitably named custom group.
Better than editing the database I think.
You can use the same set of options for two different custom fields, all you need to do is when creating custom field click on 'Reuse an existing set', select the option from the drop down list and save.