Links in Storyline work then stop working

Hello

Any advice would be much appreciated...I've developed a course (a few courses) through Storyline with links to download handouts. All is fine uploads fine and works fine and then randomly links will stop working and messages saying 'Invalid URL'. I can understand this happening when it's a link to information that may have changed, e.g. to a resource on another website, etc. but this is happening on links to folders with handouts where none of the data has changed since it was published and initially working fine!  It seems to be to ages in between happening but effectively is becoming an ongoing problem.

Thanks

Kim