Anything can cause this, including the widespread DDOS attack that has recently been causing widespread connectivity issues on a good chunk of the Internet. Unfortunately, although there's a "fix" for this, the cure is much worse than the disease.
If the server sends tasks to your computer but they don't get to your computer for any reason, the tasks will stay in limbo until they time out. There's nothing that can be done about this, sorry.
My lucky number is 75898^524288+1
Please do not PM me with support questions. They will usually go unanswered. Ask on the forums instead. Thank you!