A single 21 GB TIFF file is huge, especially if that's the compressed size, considering that FME has to decompress the file to working memory.

Have you tried using the Tiler to split the output into smaller ECW files?

What version of FME are you using? It would be good to run with FME2020 or newer, because that is when we last updated the ECW format. If you are not using a new FME, then you should upgrade to a new version.

If you are already using 2020 or newer, please try on a smaller file if possible and see if that works. But for an error like this, you should always contact our support team anyway (at safe.com/support or contact your local reseller) because this is not expected behaviour and we will want to try to fix the problem.