Two days ago my cache disk was "unmountable no file system" i mount it readonly, copy files in it and formatting resolved that issue.

May 17 21:24:24 Tower root: mount: /mnt/cache: wrong fs type, bad option, bad superblock on /dev/sdc1, missing codepage or helper program, or other error.
May 17 21:24:24 Tower kernel: BTRFS error (device sdc1): parent transid verify failed on 997179392 wanted 2695658 found 2690518
May 17 21:24:24 Tower kernel: BTRFS warning (device sdc1): failed to read tree root
May 17 21:24:24 Tower kernel: BTRFS error (device sdc1): open_ctree failed
May 17 21:24:24 Tower emhttpd: shcmd (59): exit status: 32
May 17 21:24:24 Tower emhttpd: /mnt/cache mount error: No file system

Today it is faulty again, now it is mounted but seems read only.

May 18 19:52:26 Tower kernel: ata4.00: exception Emask 0x50 SAct 0x10000000 SErr 0x90a02 action 0xe frozen
May 18 19:52:26 Tower kernel: ata4.00: irq_stat 0x00400000, PHY RDY changed
May 18 19:52:26 Tower kernel: ata4: SError: { RecovComm Persist HostInt PHYRdyChg 10B8B }
May 18 19:52:26 Tower kernel: ata4.00: failed command: WRITE FPDMA QUEUED
May 18 19:52:26 Tower kernel: ata4.00: cmd 61/a0:e0:60:3c:00/00:00:00:00:00/40 tag 28 ncq dma 81920 out
May 18 19:52:26 Tower kernel:         res 40/00:e0:60:3c:00/00:00:00:00:00/40 Emask 0x50 (ATA bus error)
May 18 19:52:26 Tower kernel: ata4.00: status: { DRDY }
May 18 19:52:26 Tower kernel: ata4: hard resetting link
May 18 19:52:27 Tower kernel: ata4: SATA link up 3.0 Gbps (SStatus 123 SControl 300)
May 18 19:52:27 Tower kernel: ata4.00: configured for UDMA/133
May 18 19:52:27 Tower kernel: ata4: EH complete

May 19 03:46:01 Tower kernel: ata4.00: exception Emask 0x50 SAct 0x3ffc SErr 0x90a02 action 0xe frozen
May 19 03:46:01 Tower kernel: ata4.00: irq_stat 0x08000000, interface fatal error
May 19 03:46:01 Tower kernel: ata4: SError: { RecovComm Persist HostInt PHYRdyChg 10B8B }
May 19 03:46:01 Tower kernel: ata4.00: failed command: WRITE FPDMA QUEUED
May 19 03:46:01 Tower kernel: ata4.00: cmd 61/80:10:40:fd:79/00:00:04:00:00/40 tag 2 ncq dma 65536 out
May 19 03:46:01 Tower kernel:         res 40/00:18:20:fe:79/00:00:04:00:00/40 Emask 0x50 (ATA bus error)
May 19 03:46:01 Tower kernel: ata4.00: status: { DRDY }
May 19 03:46:01 Tower kernel: ata4.00: failed command: WRITE FPDMA QUEUED

May 19 03:49:02 Tower kernel: BTRFS error (device sdc1): parent transid verify failed on 7798784 wanted 4229 found 4214
May 19 03:49:02 Tower kernel: BTRFS: error (device sdc1) in btrfs_run_delayed_refs:2935: errno=-5 IO failure
May 19 03:49:02 Tower kernel: BTRFS info (device sdc1): forced readonly
May 19 03:49:28 Tower kernel: loop: Write error at byte offset 5484916736, length 4096.
May 19 03:49:28 Tower kernel: print_req_error: I/O error, dev loop2, sector 10712728
May 19 03:49:28 Tower kernel: BTRFS error (device loop2): bdev /dev/loop2 errs: wr 1, rd 0, flush 0, corrupt 0, gen 0
May 19 03:49:28 Tower kernel: loop: Write error at byte offset 5484924928, length 4096.
May 19 03:49:28 Tower kernel: print_req_error: I/O error, dev loop2, sector 10712744
May 19 03:49:28 Tower kernel: BTRFS error (device loop2): bdev /dev/loop2 errs: wr 2, rd 0, flush 0, corrupt 0, gen 0
May 19 03:50:43 Tower kernel: loop: Write error at byte offset 5484916736, length 4096.
May 19 03:50:43 Tower kernel: print_req_error: I/O error, dev loop2, sector 10712728
May 19 03:50:43 Tower kernel: BTRFS error (device loop2): bdev /dev/loop2 errs: wr 3, rd 0, flush 0, corrupt 0, gen 0
May 19 03:50:43 Tower kernel: loop: Write error at byte offset 5500923904, length 4096.
May 19 03:50:43 Tower kernel: print_req_error: I/O error, dev loop2, sector 10743992
May 19 03:50:43 Tower kernel: BTRFS error (device loop2): bdev /dev/loop2 errs: wr 4, rd 0, flush 0, corrupt 0, gen 0

Is it the time to replace the disk or is there anything i can do?

tower-diagnostics-20200519-1232.zip

That looks more like a connection problem, try replacing both cables on the SSD.