> Reallocated sector count is 880. The temperature stays above 50°C. Is this a hard drive failure?

880 reallocated sectors is a sufficient reason to replace the dying disk.

For a rotating hard drive 50°C is already dangerously near the maximum allowed temperature. That temperature is increasing wear.

What is your ambient temperature?
You should provide cooling for your drive to make your disk approach a hopefully much lower ambient temperature instead of heading towards the maximum allowed temperature.

If a possible successor of your drive is being put in that thermal environment, he won't live long.

> Try to format the disk using a slow (not quick) format, which will rewrite all the sectors. Check the

Any variation of a window format command will delete stuff on the drive.
This is a irresponsible hint!

>  Is the Reallocated Sector Count high? 

It is sufficiently high to justify replacement.

Read this SMART report to see how high those figures can get.
https://forum.cgsecurity.org/phpBB3/viewtopic.php?t=12261

> There's no SMART warning. The drive is less than 3 years old. 

Don't expect any. If it happens to you again, check the SMART parameters regularly and you can watch the process of increasing figures.