pinochico and GJC replied while I was typing this. but maybe I can clarify the procedure a little bit further...

At present
Maximum coupon usage limit reached, please try different code.
is hard-coded in the file ...\components\com_virtuemart\helpers\cart.php. line 1130, which means it is not translatable in the de-DE.com_virtuemart.ini file.
In order to make it translatable, you would have to replace that with something like:
return vmText::_('COM_VM_COUPON_MAX_LIMIT_REACHED');
and afterwards add
COM_VM_COUPON_MAX_LIMIT_REACHED="Maximum coupon usage limit reached, please try different code."
to the frontend en-GB.com_virtuemart.ini file and maybe
COM_VM_COUPON_MAX_LIMIT_REACHED="Maximale Coupon-Verwendungsgrenze erreicht, bitte anderen Code ausprobieren.."
to the German frontend de- DE.com_virtuemart.ini file.
A similar change will be needed for line 1201 in cart.php
...or wait for the next release of VirtueMart. It should be corrected there.