This problem has been reproduced on a system where the creation of a new system account takes long, logs report more than 18 seconds:

3208204 3208204 2023-02-28 17:08:05 190.936 NXSERVER Process '/usr/NX/bin/nxexec /usr/NX/scripts/restricted/nxuseradd.sh guest0021   1004 ' with pid '3208407/3208407' finished with exit code 0 after 18,658 seconds.

In this context, if a second user requests for a new guest account while the previous account is still in process to be created, the same account (in the example above guest0021) is assigned to both users:

3208204 3208204 2023-02-28 17:08:06 600.794 NXSERVER NXMsg: Sent response message 'NX> 103 Hello on board guest0021.'
3209728 3209728 2023-02-28 17:08:07 639.919 NXSERVER NXMsg: Sent response message 'NX> 103 Hello on board guest0021.'

This leads to permission problems when accessing home directory:
> 3209728 3209728 2023-02-28 17:08:08 681.457 NXSERVER Read 'NX> 596 Error: Cannot create /home/guest0021/.nx. Error is 'Permission denied'.\n' 80 from FD#24.
> 3208204 3208204 2023-02-28 17:09:17 443.351 NXSERVER Read 'NX> 596 Error: Cannot create /home/guest0021/.nx. Error is 'Permission denied'.\n' 80 from FD#24.