Qsys (Platform Designer) seq: add_fileset_file: No such file .. hps_AC_ROM.hex

I have a similar issue with 20.1. I'm trying to build a project which builds fine in 18.1, at the Qsys generation stage I get the following errors:

Error: border: Error during execution of script generate_hps_sdram.tcl: seq: Error during execution of "{C:/intelfpga/20.1/quartus/../nios2eds/Nios II Command Shell.bat} make all 2>> stderr.txt": child process exited abnormally

Error: border: Error during execution of script generate_hps_sdram.tcl: seq: Execution of command "{C:/intelfpga/20.1/quartus/../nios2eds/Nios II Command Shell.bat} make all 2>> stderr.txt" failed

Error: border: Error during execution of script generate_hps_sdram.tcl: seq: child process exited abnormally

Error: border: Error during execution of script generate_hps_sdram.tcl: seq: add_fileset_file: No such file C:/Users/JONATH~1/AppData/Local/Temp/alt8432_5628436629018323475.dir/0004_seq_gen/hps_AC_ROM.hex

Error: border: Error during execution of script generate_hps_sdram.tcl: Generation stopped, 3 or more modules remaining

Error: border: Execution of script generate_hps_sdram.tcl failed

I have ensured the TEMP variables are defined as local, on the C drive. Not sure why it has JONATH~1 in the path.

I am on Windows 10.0.18362