These symptoms are common when attempting to use serial printer software with a USB printer.
Manufacturers, such as Epson America, frequently provide both kinds of software for their USB printers that support both kinds of connections.
Though Macintosh computers with USB ports, such as the iMac and Power Macintosh G3 (Blue and White), do not have serial ports, serial printer software can nevertheless be installed.
Users are advised to carefully examine the instructions accompanying their printer software to ensure that it is appropriate for a USB printer connection, not a serial one. If in doubt, contact the manufacturer, who will typically make both types of printer software available on the Internet. Be careful to download only the USB version, not the serial version that may be listed adjacent to it.
Important: The serial port in use alert is a very good sign that the wrong type of printer software is in use, but the other alerts may also stem from other causes, such as damaged, missing, or partially installed printer software.