Email subject when order is done contains =?utf-8?B? and a lot of numbers after. I found the file public_html/components/com_virtuemart/helpers/shopfunctions which occurs the problem.
At line 877 of this file we have this code $subjectMailer= '=?utf-8?B?'.base64_encode($subject).'?='; that is not executing properly. To workaround with it i have to comment this line of code and the it works fine
// $subjectMailer= '=?utf-8?B?'.base64_encode($subject).'?='; //get rid of this code and put the subject clear on the variable
$subjectMailer= ($subject);
I confirm this issue. I solved it the same way. But obviously this is wrong. In VM3 such code worked.
Luckily, the code works on j4 and j3