Hi,
The problem seems to be caused by the insufficient RAM size.
1. Try using the latest version of TestComplete (ver. 7.52) that has some improvements in the HTTP Load Testing feature.
2. Make sure that before TestComplete is launched, there are at least 2 Gb of free RAM available. If possible, try increasing the RAM size on the PC to achieve this.
3. Try reducing the amount of virtual users involved in the test.
4. Try splitting the Load Testing task into several smaller tasks and execute them as individual Test Items.