Having this crash when someone use the Filler in the nether, this does not occuer everytime, but, sometimes crash the server :C

Do you know what size the filler was set up with? And what height the filler was set up at?

in my server, the markers are limited to 21blocks of distance

Not the size, but the heigh it was in the nether "224" as mentioned in the crashreport.

687,224,25

The players had bypassed the nether upper bedrock