we can use JasperReports® Server 6.4.2 or 6.4.3 with Spectrum 10.2.3 if we patch the SpectroSERVER.
PTF_10.2.316 for 6.4.2
PTF_10.2.336 for 6.4.3.
This would have the double benefit of future proofing the JasperServer as it will be compatible with Spectrum 10.3.
However, we got the same problem using the 6.4.2.
We then saw that disk space was very low on the VM and increasing it resolved the problem.