"Error: Cannot hold value outside clock edge" ?

I again wonder if you have more than one clock in the process. Your elsif line is checking for the falling edge of RS232InInt, making that signal be a clock, but your first error message is about trying to use "S" as a clock.

The register examples in the coding guidelines have nothing else ANDed in the "if" that does the clock edge detection.

Another place for code examples is in the Quartus text editor at "Edit --> Insert Template". There are full designs containing registers, and there are multiple register examples in the "Insert Template" dialog box at "VHDL --> Logic --> Registers".