Is there a way to un-abort a task?
I accidentally aborted it, it is still in my task list.

Also, when my pc entered sleep mode, Boinc displayed many computation errors; probably due to the sleep state.
Is there a way to redo the task?

Simple answer NO.

Simple answer NO.

More complex answer: Still NO if your computer has communicated with the project(s) since aborting task(s.)

If you have a backup of the boinc data directory from before your computer produced the errors it is possible and there is even a complex way of doing it when some tasks have errors and others don't. CPDN used to have instructions on the forum for restoring tasks from backups but it isn't worth it these days. I did it successfully a couple of times in the days when CPDN tasks could last four or five months on a computer that was then reasonably fast.

So while still possible it isn't worth the hassle. Even on CPDN tasks that can last the best part of a month on slower computers, the only errors that might benefit from this are those that occur due to power failure or closing computer down during check pointing. Other errors will almost certainly occur at the same point in computation.

Also, when my pc entered sleep mode, Boinc displayed many computation errors; probably due to the sleep state.

A biot surprised if this is the reason. I have yet to find a task crashing as a result of sleep or hibernate, the latter being disabled by default on recent Ubuntu releases.) In contrast until recently,with Linux tasks, they would often crash when machine was switched off for any reason, even if computation suspended and BOINC closed down first. It would occasionally happen with Windows tasks running under WINE but not nearly as often. Re-starting following a kernel update seemed to be the worst case. (Don't know if this affected projects other than CPDN but if I was only running them I would do the reboot when I only had unstarted tasks in the queue.