Hi!
I'v hit a BUG ON during btrfs check:
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server:~# btrfs check --progress --repair /dev/sde
enabling repair mode
Opening filesystem to check...
Checking filesystem on /dev/sde
UUID: d5fa971b-6546-424d-87c1-dcd688eacdac
[1/7] checking root items                      (0:59:38 elapsed, 
104205418 items checked)
Fixed 0 roots.
[2/7] checking extents                         (3:12:27 elapsed, 5[2/7] 
checking extents                         (3:12:28 elapsed, 5234ref 
mismatch on [30539776 16384] extent item 0, found 1 elapsed, 7917478 
items checked)
tree backref 30539776 parent 2 root 2 not found in extent tree
backpointer mismatch on [30539776 16384]
adding new tree backref on start 30539776 len 16384 parent 0 root 217548 
items checked)
btrfs unable to find ref byte nr 693215772672 parent 0 root 2  owner 1 
offset 0checked)
transaction.c:168: btrfs_commit_transaction: BUG_ON `ret` triggered, 
value -5
btrfs(+0x3b748)[0x562bc6163748]
btrfs(btrfs_commit_transaction+0x53)[0x562bc6163af5]
btrfs(+0x554bc)[0x562bc617d4bc]
btrfs(cmd_check+0x1288)[0x562bc617ee75]
btrfs(main+0x1f3)[0x562bc613be63]
/lib/x86_64-linux-gnu/libc.so.6(__libc_start_main+0xf5)[0x7fc93c23bb45]
btrfs(_start+0x2a)[0x562bc613beaa]
Abgebrochen
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Kernel: 4.19.0-2-amd64 #1 SMP Debian 4.19.16-1 (2019-01-17) x86_64 GNU/Linux

btrfs-progs v4.20.1

After that I mounted the fs and got this:
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[54682.472317] BTRFS info (device sdf): enabling inode map caching
[54682.472321] BTRFS info (device sdf): disk space caching is enabled
[54682.472324] BTRFS info (device sdf): has skinny extents
[54710.634379] BTRFS info (device sdf): checking UUID tree
[54776.365908] BTRFS error (device sdf): parent transid verify failed on 
30539776 wanted 695635 found 695646
[54776.365916] BTRFS error (device sdf): parent transid verify failed on 
30539776 wanted 695635 found 695646
[54780.325562] BTRFS error (device sdf): parent transid verify failed on 
30539776 wanted 695635 found 695646
[54780.325567] BTRFS error (device sdf): parent transid verify failed on 
30539776 wanted 695635 found 695646
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/dev/sde, ID: 2
    Device size:             3.64TiB
    Device slack:              0.00B
    Data,RAID1:              2.29TiB
    Metadata,single:       800.00MiB
    Metadata,RAID1:        124.00GiB
    System,RAID1:            8.00MiB
    Unallocated:             1.23TiB

/dev/sdf, ID: 1
    Device size:             3.64TiB
    Device slack:              0.00B
    Data,single:             8.00MiB
    Data,RAID1:              2.29TiB
    Metadata,single:       816.00MiB
    Metadata,RAID1:        124.00GiB
    System,single:           4.00MiB
    System,RAID1:            8.00MiB
    Unallocated:             1.23TiB