Linux unmount automatically external drive / array after time

I have an external hard drive array that is permanently attached to the computer. I specified the array in /etc/fstab. When I start the computer, it is also mounted correctly. Only after a while it is automatically unmount. I don't want this.

If i use mount -a, it is mounted correctly again and works fine until the next unmount.

[fstab]

UUID=46c61289-7e60-4059-80c4-9322cdccbd5f  /mnt/backup-raid       ext4 defaults 0 0

Until about 4 weeks ago I had no problems with it.

logfile

Nov 27 21:17:01 - CRON[132829]: pam_unix(cron:session): session opened for user root(uid=0) by (uid=0)
Nov 27 21:17:01 - CRON[132830]: (root) CMD (   cd / && run-parts --report /etc/cron.hourly)
Nov 27 21:17:01 - CRON[132829]: pam_unix(cron:session): session closed for user root
Nov 27 21:19:59 - kernel: usb 2-1: USB disconnect, device number 5
Nov 27 21:19:59 - kernel: blk_update_request: I/O error, dev sde, sector 0 op 0x1:(WRITE) flags 0x800 phys_seg 0 prio class 0
Nov 27 21:19:59 - kernel: sd 9:0:0:0: [sde] Synchronizing SCSI cache
Nov 27 21:19:59 - systemd[1]: Unmounting /mnt/backup-raid...
Nov 27 21:19:59 - systemd[99933]: mnt-backup\x2draid.mount: Succeeded.
Nov 27 21:19:59 - kernel: blk_update_request: I/O error, dev sde, sector 0 op 0x1:(WRITE) flags 0x800 phys_seg 0 prio class 0
Nov 27 21:19:59 - kernel: blk_update_request: I/O error, dev sde, sector 0 op 0x1:(WRITE) flags 0x800 phys_seg 0 prio class 0
Nov 27 21:19:59 - kernel: blk_update_request: I/O error, dev sde, sector 0 op 0x1:(WRITE) flags 0x800 phys_seg 0 prio class 0
Nov 27 21:19:59 - kernel: blk_update_request: I/O error, dev sde, sector 0 op 0x1:(WRITE) flags 0x800 phys_seg 0 prio class 0
Nov 27 21:19:59 - kernel: blk_update_request: I/O error, dev sde, sector 0 op 0x1:(WRITE) flags 0x800 phys_seg 0 prio class 0
Nov 27 21:19:59 - kernel: blk_update_request: I/O error, dev sde, sector 3905161216 op 0x1:(WRITE) flags 0x800 phys_seg 1 prio class 0
Nov 27 21:19:59 - kernel: blk_update_request: I/O error, dev sde, sector 3905161216 op 0x1:(WRITE) flags 0x800 phys_seg 1 prio class 0
Nov 27 21:19:59 - kernel: Buffer I/O error on dev sde1, logical block 488144896, lost sync page write
Nov 27 21:19:59 - kernel: JBD2: Error -5 detected when updating journal superblock for sde1-8.
Nov 27 21:19:59 - kernel: Aborting journal on device sde1-8.
Nov 27 21:19:59 - kernel: blk_update_request: I/O error, dev sde, sector 3905161216 op 0x1:(WRITE) flags 0x800 phys_seg 1 prio class 0
Nov 27 21:19:59 - kernel: blk_update_request: I/O error, dev sde, sector 3905161216 op 0x1:(WRITE) flags 0x800 phys_seg 1 prio class 0
Nov 27 21:19:59 - kernel: Buffer I/O error on dev sde1, logical block 488144896, lost sync page write
Nov 27 21:19:59 - kernel: JBD2: Error -5 detected when updating journal superblock for sde1-8.
Nov 27 21:19:59 - kernel: EXT4-fs error (device sde1): ext4_put_super:1194: comm umount: Couldn't clean up the journal
Nov 27 21:19:59 - kernel: EXT4-fs (sde1): Remounting filesystem read-only
Nov 27 21:19:59 - systemd[1]: mnt-backup\x2draid.mount: Succeeded.
Nov 27 21:19:59 - systemd[1]: Unmounted /mnt/backup-raid.
Nov 27 21:19:59 - kernel: sd 9:0:0:0: [sde] Synchronize Cache(10) failed: Result: hostbyte=DID_ERROR driverbyte=DRIVER_OK
Nov 27 21:20:00 - kernel: usb 2-1: new SuperSpeed Plus Gen 2x1 USB device number 6 using xhci_hcd
Nov 27 21:20:00 - kernel: usb 2-1: New USB device found, idVendor=174c, idProduct=55aa, bcdDevice= 1.00
Nov 27 21:20:00 - kernel: usb 2-1: New USB device strings: Mfr=2, Product=3, SerialNumber=1
Nov 27 21:20:00 - kernel: usb 2-1: Product: IB-RD3802-C31
Nov 27 21:20:00 - kernel: usb 2-1: Manufacturer: ICY BOX
Nov 27 21:20:00 - kernel: usb 2-1: SerialNumber:
Nov 27 21:20:00 - kernel: scsi host9: uas
Nov 27 21:20:00 - kernel: scsi 9:0:0:0: Direct-Access     IB-      RD3802-C31 RAID1 0    PQ: 0 ANSI: 6
Nov 27 21:20:00 - kernel: sd 9:0:0:0: Attached scsi generic sg4 type 0
Nov 27 21:20:00 - kernel: sd 9:0:0:0: [sde] 7813988353 512-byte logical blocks: (4.00 TB/3.64 TiB)
Nov 27 21:20:00 - kernel: sd 9:0:0:0: [sde] 4096-byte physical blocks
Nov 27 21:20:00 - kernel: sd 9:0:0:0: [sde] Write Protect is off
Nov 27 21:20:00 - kernel: sd 9:0:0:0: [sde] Mode Sense: 43 00 00 00
Nov 27 21:20:00 - kernel: sd 9:0:0:0: [sde] Write cache: enabled, read cache: enabled, doesn't support DPO or FUA
Nov 27 21:20:00 - kernel: sd 9:0:0:0: [sde] Optimal transfer size 33553920 bytes not a multiple of physical block size (4096 bytes)
Nov 27 21:20:00 - kernel:  sde: sde1[/CODE]

Thanks for your help.