The default is port 8080. You can change the port by editing the systemd service at the following location:
This can happen any number of ways, such as with an SD card that is bad, or if you shut your system down while a commit or push is in progress, or any number of other ways.
You can find out what the real issue is by viewing the log file.
Open the terminal in the webide and execute the following: cd /usr/share/adafruit/webide/logs
Then execute the following command to view the last few commands from the log file: cat output.log
You can also view the errors.log file: cat errors.log
If you see an error such as:
error: object file .git/objects/2c/e98048b95181870dbba33675f3d2bc1d995c47 is empty
fatal: loose object 2ce9804 (stored in .git/objects/2c/e98048b95181870dbba33675f3d2bc1d995c47) is corrupt
You can try fixing that by following this fix from Stack Overflow.