Same problem here, I just asked one of my users (they've never reported it to me though).
Dell R720 servers w/ dual E5-2665 and 256GB RAM
View 5.3
ESXi 5.5 - 1331820
vCSA 5.5.0.5201 Build 1476389
APEX 2800 LP Cards using the 2.3.3 driver
Also thinking the APEX card could be related to this issue as well: Re: View 5.3 - Another task is already in progress