It looks like an issue with encryption, you might have not entered the right password
the problem with that is, and ive seen this plenty of times, is that TWRP stops prompting you for the password to decrypt
not even manually entering is via command like (with adb) like this decrypts it
abd shell twrp decrypt <password>
its at this point ill tell you the quickest way to resolve things is to use format data (or using fastboot:
fastboot erase userdata
) and start over.
Ive never found a workaround to bypass having to format userdata, if someone else has, id love to hear it
Top tip: always keep a copy of whats on your internal sdcard...i always back mine up before flashing a new ROM
As for backing up app and app data....migrate (in my sig)
My ROM flash process (10 second version):
migrate backup
backup files on sdcard
flash ROM
reboot
flash GApps (if necessary)
flash magisk
reboot
setup ROM
reboot
migrate restore
restore sdcard files (if ive had to erase data)