Your ciphertext consists of 40 hexadecimal digits, which encode 20 bytes. Your plaintext is 20 ASCII characters long. The obvious assumption, therefore, is that the ciphertext has been calculated by taking the ASCII code of each plaintext character and either adding (modulo 256) or XORing a corresponding key byte with it.
Furthermore, the bytes in your ciphertext are not uniformly distributed over the range from 0 to 255, suggesting that what you have is not a secure stream cipher but something weaker.
Indeed, taking each byte of the ciphertext and subtracting the corresponding byte of plaintext yields:
42 22 28 89 2b 50 48 5b 28 61 7b 8c e7 27 70 24 46 74 23 50
or, in ASCII (using
\xHH for characters outside the valid ASCII range):
Those extra characters might belong to some 8-bit extension of ASCII, such as Windows-1252, which would make the "keystream" look like this:
This looks plausibly like something someone might produce simply by mashing "random" keys on a non-English keyboard, so it's at least plausible that the "encryption" mightsimply consist of adding this key string to the plaintext byte by byte. However, without more samples, there's no real way to confirm this.