I have encountered this problem while trying to setup osTicket in a docker container..
I have resolved it!
Here is what I did...
Assuming the container name is "osTICKET" Run the following commands to fix it...
Copy the file out of the container...
docker cp osTICKET:/usr/local/src/osticket/setup/inc/class.installer.php class.installer.php
Edit the "class.installer.php" file and find the line with the following on it.
$this->errors['username']=__('Bad username');
Then remark out that line and the one above it by putting "//" (no quotes) at the front of the lines.
Copy it back: docker cp class.installer.php osTICKET:/usr/local/src/osticket/setup/inc/class.installer.php
Remove the old file: rm class.installer.php
Start the container.
Install should work now. If you modify the container the file will reset back to its original state.