Hi there,
I had the same issue when updating the premium version of the Timely "All in One Calender" from version 1.8.4 to 1.9. This is a very well documented and maintained plugin.
It seems that both the Pagelines Framework and the new calendar version are using the LESS thingy. There is a conflict between both uses of it.
Given the error message above, when Pagelines tries to use some of the classes of LESS, they have already been instantiated by the calendar. or vice versa. At least one plugin need to check whether the LESS thingy has already been loaded.
Some people suggest to use autoload (whatever that is; you will probably know):
See:
Please Login or Register to see this Hidden Content
See also the Timely calendar forum for a similar error description there:
Please Login or Register to see this Hidden Content
Please help with that issue as the calendar is widely used. Deinstalling or not updating that plugin is definitely NOT an option.