You may need to do as suggested above here, the forum elsewhere,and by Ivar, the administrator.
If you do a new achievement,secret achievement, and your maximum hero energy drops, just remove equipment and put it back on,or do this anyway, if its some other reason your hero lost its maximum energy.
If it doesn't work,its a different problem perhaps.
Dear Jarls,
Thank you for your messages.
We apologize for any inconvenience caused and want to assure you that your request is already being processed. It will be resolved as soon as possible. If you have any additional information, please provide us with it.
Right now, as said KROW before, you can take off and equip any Hero’s equipment and Hero's Energy will be restored.
We hope for your understanding.