FCP 'share' file export randomly failing

Error -536870181 converted to hexadecimal is 0xE00002DB. That is defined in the open source file IOReturn.h, which is part of the IOKit framework used in macOS. There we find:

#define kIOReturnNoSpace iokit_common_err(0x2db) // no space for data

It implies some kind of out-of-space condition during an I/O operation. That said, it might not be a normal "out of space" condition, e.g, it could mean not enough memory in the kernel to allocate a buffer for the data. The error is not specific.

Use Activity Monitor to check memory pressure and memory consumption by FCP and VTEncoderXPCService during export. That is an external "helper" process that does much of the encoding. After the error, use the Console app to check the MacOS system log and diagnostic reports corresponding to that time.

If that does not reveal anything, use terminal to set up a command to gather the previous 60 sec of MacOS system log immediately after the export fails. Syntax:

sudo log show --last 60s > ~/Documents/LogShow60s.txt

It will prompt you for your system password, so you might practice this command in advance. Once the FCP export happens, run this command (already have it entered, except for pressing Enter). Provided the error happened within the prior 60 sec, the data will be relevant. The output is very verbose, so we only want to examine a short time period.

You can zip that and upload to a file sharing service I can download from.

As Luis said, trying the export to a local drive might help understand the problem.

I've had occasional failures to write to our QNAP NAS if I'm trying to overwrite a file. I could always work around it by renaming the file in the export dialog so there was no overwrite.

You could try, as a test, exporting a few of these files to your internal drive. This would let you know if somehow the problem is related to the NAS, or the network.