grid.arrange from gridExtras exiting with "only 'grobs' allowed in 'gList'" after update

It is because grid.arrange does not have a main parameter anymore (seems to have been eliminated around Summer 2015 with the 2.0.0 release) and thus thinks that the main= parameter must be a grob. To replace the main, you can use the top parameter now (as well as a bottom, left, and right).

So this works for example:

library(ggplot2)
p1 = qplot(1:10,rnorm(10))
p2 = qplot(1:10,rnorm(10))

library(gridExtra)
grid.arrange(p1, p2, ncol=2,top="Main Title")

The message is a bit confusing, that is because it looks at all the parameters it does not know and assumes they might be grobs (graphical objects) that it can plot. A confusing error message like this is the price you pay for that flexibility.

Note: - if you have a lot of grobs you should consider packing them all into a list and use the form:

grid.arrange( grobs = list(p1,p2,...),...

Here is what that above code results in: