If you suspend a task on your first GPU (for example to run a beta) and allow another task to complete on a different GPU, you will not download any new tasks (as you have suspended one task).
Should you then resume the suspended task, it will try to run on the available GPU!
As it was initially running on the first GPU it will error when it tries to start on a different GPU.

This occurs when you have Boinc configured to suspend tasks and keep them in memory.
This might not happen if suspended tasks are saved to disk.

So, don’t suspend one task at a time!
If you have to, suspend the project or just use Snooze GPU.