This article will explain you the cause(s) and solution(s) for this console error.
Error in console:
INFO Error: Unable to access jarfile /custom.jar
The custom.jar file cannot be found.
Make sure the jar file you want to use is named custom.jar and is placed in the base (/) directory of your server. If you don't want to use a Custom JAR, change the JAR File option in Multicraft to the JAR you want to use.