Troubleshoot - Debian system not suspending

The below is shown in the virtual console:

Jan 08 18:24:04 dom kernel: Freezing user space processes failed after 20.006 seconds (1 tasks refusing to freeze, wq_busy=0):

Searching the sytemd logs reveals more details, including the line fuse_simple_request which points to the FUSE layer.
Use journalctl -r to list the most recent log entries, and the / key followed by freeze to search for the relevant log entry.

Jan 08 18:24:24 dom kernel: Freezing user space processes failed after 20.006 seconds (1 tasks refusing to freeze, wq_busy=0):
Jan 08 18:24:24 dom kernel: Freezing user space processes
Jan 08 18:24:04 dom kernel: Filesystems sync: 0.012 seconds
Jan 08 18:24:04 dom kernel: PM: suspend entry (s2idle)
Jan 08 18:24:04 dom kernel: PM: suspend exit
[..]
Jan 08 18:24:04 dom kernel:  </TASK>
Jan 08 18:24:04 dom kernel: R13: 000056426d10c080 R14: 000056426c6d4100 R15: 000056426d10cc80
[..]
Jan 08 18:24:04 dom kernel: RIP: 0033:0x7fddceb1e52a
Jan 08 18:24:04 dom kernel:  entry_SYSCALL_64_after_hwframe+0x76/0x7e
[..]
Jan 08 18:24:04 dom kernel:  vfs_fstatat+0x80/0xb0
Jan 08 18:24:04 dom kernel:  vfs_statx+0xa3/0x100
Jan 08 18:24:04 dom kernel:  vfs_statx_path+0x9f/0xe0
-----------------------
Jan 08 18:24:04 dom kernel:  fuse_do_getattr+0xf1/0x2c0
Jan 08 18:24:04 dom kernel:  fuse_simple_request+0x17e/0x2c0
-----------------------
Jan 08 18:24:04 dom kernel:  ? __pfx_autoremove_wake_function+0x10/0x10
Jan 08 18:24:04 dom kernel:  request_wait_answer+0xd0/0x2a0
Jan 08 18:24:04 dom kernel:  schedule+0x27/0xf0
Jan 08 18:24:04 dom kernel:  __schedule+0x3ec/0xb00
Jan 08 18:24:04 dom kernel:  <TASK>
Jan 08 18:24:04 dom kernel: Call Trace:
Jan 08 18:24:04 dom kernel: task:df              state:D stack:0     pid:28146 tgid:28146 ppid:28145  flags:0x00004006
Jan 08 18:24:04 dom kernel: Freezing user space processes failed after 20.006 seconds (1 tasks refusing to freeze, wq_busy=0):

It is the file system in user space (FUSE) that uses the SSH File Transfer Protocol (SFTP) to mount a remote file system.
In the /etc/fstab there is a mountpoint wich uses sshfs to mount a remote filesystem.

admin@ec2-18-xxx-xxx-xxx.us-west-2.compute.amazonaws.com:/mnt/remote   /media/user/sshfs1 fuse.sshfs defaults,port=22,IdentityFile=/home/user/my-keypair1.pem,allow_other,default_permissions,reconnect 0

Unmounting the sshfs mount and disabling this line will make the system suspend sucessfully.
Below is the relevant log for the systemd-sleep unit which is used for suspending the machine.

Jan 08 18:23:44 dom systemd-sleep[28484]: Failed to freeze unit 'user.slice': Connection timed out
Jan 08 18:23:44 dom systemd-sleep[28484]: Performing sleep operation 'suspend'...
Jan 08 18:24:24 dom systemd-sleep[28484]: Failed to put system to sleep. System resumed again: Device or resource busy
Jan 08 18:24:24 dom systemd-sleep[28484]: Successfully thawed unit 'user.slice'.
Jan 08 18:24:24 dom systemd[1]: systemd-suspend.service: Main process exited, code=exited, status=1/FAILURE
Jan 08 18:24:24 dom systemd[1]: systemd-suspend.service: Failed with result 'exit-code'.
Jan 08 18:24:24 dom systemd[1]: Failed to start systemd-suspend.service - System Suspend.
Jan 08 18:24:24 dom systemd[1]: systemd-suspend.service: Consumed 2.281s CPU time, 5.9M memory peak.