I had a similar problem where I are getting this message every time I tried to save: "*** Problems were found that could not be fixed! *** The name for CMaterial (290526) is not valid". It would still let me save the file though. I was able to solve the problem by opening the materials window and hovering over each material to look at the name. I found 3 materials that somehow had blank names. After renaming those materials the problem went away.
I don't know how those blank named materials got there in the first place though, I tried to recreate the problem but SketchUp won't let you name a material with a blank name.