Problem with create directory wp-content/uploads/2013/04 on my Multisite installation

Hi Support,

just set up Multisite on Server and seems to be running fine – had previous plug-ins installed and are all network activated. Just went to upload your plugin dashboard and got the following message…

Unable to create directory wp-content/uploads/2013/04. Is its parent directory writable by the server?….

I’ve made sure that all directories are CHMOD values 755 & files 644.

I’m using the following plug-ins ( as recommended by a Lynda.com tutorial securing WordPress sites)

Login Security Solution Version 0.37.0 | By Daniel Convissor

ThreeWP Activity Monitor Version 2.11 | By edward mindreantre

WordPress File Monitor Version 2.3.3 | By Matt Walters

Any ideas what the problem could be. Is there a further bit of wordpress set-up needed or something server side. Or is it a problem that using these security plug-ins poises.

Thanks for any help with this.