Nostalrius Begins - Quality wow vanilla realm (1.12)

Hello together!

I just ran around in Trisfal Glades to skill up my herbalism. Suddenly I got a DC and when I wanted to relog it says 'Invalid username or password'. After 10 - 15 minutes I was able to login again (with same login information).

I just want to ask for the future, is that a common bug or something else?

Sounds like a temporary server crash. Those are quite rare since the team has been actively fixing problems that result in server crashes. Is that the first time you've experienced that? If so I wouldn't worry about it.