Cant mount userdata after interrupted encryption.

[CatInATree]

I have a Mi5s plus
After flashing a new rom and rebooting I was prompted to encrypt the phone. I did not want to do so. I interupted before giving a password.
Now I can't mount userdata even temporarily (im using 'adb shell')
I need the diskspace to transfer my backups to the phone (I use TWRP)

Note: sector size is 4096 (not 512)
Found valid GPT with protective MBR; using GPT

Disk /dev/block/sda: 240549888 sectors, 1664M
Logical sector size: 4096
Disk identifier (GUID): 98101b32-bbe2-4bf2-a06e-2bb33d000c20
Partition table holds up to 16 entries
First usable sector is 6, last usable sector is 30068730

Number Start (sector) End (sector) Size Code Name
1 6 9 16384 0700 ssd
2 10 11 8192 0700 switch
3 12 191 720K 0700 bk1
4 192 223 128K 0700 devcfg
5 224 255 128K 0700 devcfgbak
6 256 383 512K 0700 keystore
7 384 511 512K 0700 frp
8 512 1023 2048K 0700 bk2
9 1024 2047 4096K 0700 misc
10 2048 4095 8192K 0700 bk3
11 4096 8191 16.0M 0700 oops
12 8192 16383 32.0M 0700 persist
13 16384 32767 64.0M 0700 recovery
14 32768 98303 256M 0700 cache
15 98304 30068730 114G 0700 userdata

~ # mount /dev/block/sda15 /sdcard/
mount: mounting /dev/block/sda15 on /sdcard/ failed: Invalid argument

How can I mount userspace?
Any fsck I try fails
How can (and must I) format userspace to be able to mount it?
What fs format should it be? mkfs.ext2?
How will this influence the phone after restoring boot data and system? (using TWRP)
Can I save my user data somehow?

 

[CatInATree]

I could already fix this by just letting the miui ROM reset and restart. Now I have a clean MIUI installlation. Data lost unfortunately.
Would still like to know answers from curiosity


My intention was to update firmware for new Lineage OS