It appears that with Scrypt-N You need to have at least same or more amount of system RAM as is on Your VGA, so If You have 2G of ram and some R9 VGA with 3G, Youll get this error with higher thread concurrency. Eg (approximates)
R9 280X w 3G video RAM + 2GB system RAM on scrypt with TC 8192: 700khs
R9 280X w 3G video RAM + 4GB system RAM on scrypt-n with TC 8192: 350khs
R9 280X w 3G video RAM + 2GB system RAM on scrypt-n with TC 8192: Error -5: Enqueueing kernel onto command queue. (clEnqueueNDRangeKernel)
R9 280X w 3G video RAM + 2GB system RAM on scrypt-n with TC 4096: 200khs
I have 2gb of GPU ram and 6gb of system ram... so... can't imagine that's the problem. The same card is doing 250 on windows, but 191 on linux...