- why did that happen without changing any configuration?
Why did what happen? If it started working after you changed "null" to "nul" then you certainly changed the configuration.
- what's the importance of since_db file. I don't think it's mandatory specifying in a configuration file?
No, but setting it to "nul" or "/dev/null" (depending on your OS) will make Logstash oblivious to any old saved state about the current position in the file. The file input documentation has a few paragraphs on sincedb files. Please read them.
- difference between forward and backward slashes for specifying file path?
Well, it seems Ruby or some Ruby library used by Logstash works better with forward slashes, even on Windows.
- what exactly are globbed files and what is glob did not work?
A filename glob is a filename pattern that may or may not include wildcards. A glob that doesn't work is a glob that doesn't expand to any actual files. See glob (programming) - Wikipedia.