marton_1:
I have v1.5.1 with the classic template
I installed edit-order about a year ago and it worked fine.
I wanted to change an order today but the "edit" button is not showing up? ...
Sounds like files in the Zen Cart administrative folder were changed. Has anyone made ANY changes to the site?
Most of the time when something "mysteriously changes", the root cause is someone made changes to the site (restored from a backup, installed something new, edited files, removed something, attempted an upgrade, hosting provider changed the version of PHP, etc).
Best case scenario, the changes were made by an authorized party (done by you, someone you hired, or the hosting provider). Worst case scenario, the changes were caused by someone compromising the server hosting your website. Either way it would be a good idea to compare your site against a known good backup (and against the stock Zen Cart installation) as recommended by mc12345678 to verify no other "mysterious changes" exist.
marton_1:
I uploaded the edit-order files with ftp and now I have the "edit" button again ...
Glad you were able to simply re-install Edit Orders to restore the functionality. When (re-)installing Edit Orders if one overwrites files, one will need to apply all applicable bug fixes posted since the (re-)installed version of Edit Orders was released. It has been awhile since the current stable version of Edit Orders (4.1.4) was released, so some of the posted Bug Fixes may be a number of pages back in this thread.
It sounds like a fellow forum member has kindly re-posted one of the fixes applicable to your store environment (based upon the error logs).
marton_1:
... ```
[26-Dec-2015 23:17:06 Europe/Berlin] PHP Fatal error: Call to undefined function zen_product_in_category() in C:\xampp\htdocs\zencart151--admin---\includes\functions\extra_functions\edit_orders_functions.php on line 326
Based on this error, you will need to apply the bugfix for [Coupons with Category / Product Restrictions](https://www.zen-cart.com/showthread.php?200407-Edit-Orders-v4-0-Support-Thread&p=1242779#post1242779).