Troubleshooting

Not enough memory

Try either increasing the max allocated memory or decreasing max queued seeds.

Changing the maximum allocated memory

Prism / MultiMC

Change the "Maximum memory allocation" option.

Other launchers

Append -Xmx{size} to JVM args, where {size} is the upper limit, optionally followed by a unit. For example -Xmx4G sets the limit at 4 GiB. Make sure -Xmx or -XX:MaxHeapSize is only specified once.