Thank you for your answer.
Unfortunately, string you used in your example (Line 71 at Form1.cs) contains no special utf-8 characters - all of the letters are from standard ASCII set.
If you try to replace that line with some Cyrillic letters, or Eastern European letters, you will reproduce the issue:
//Add data
qrBarcode.Text = "АБВГДЂ ŠšĐđĆ掞";
Just in case this post gets messed up in transit because of unicode characters, characters are:
//Add data
qrBarcode.Text = "\u0410\u0411\u0412\u0413\u0414\u0402 \u0160\u0161\u0110\u0111\u0106\u0107\u017D\u017E";
In my tests, any of these letters wil cause an error in barcode creation.
Best regards,
Slobodan