This is quick fix for sentry generated backup.json file.

sentry_backup_fix.py

import json

FILENAME = "backup.json"
FILENAME_FIXED = "backup_fixed.json"


def restore():
    with open(FILENAME, "r") as f:
        lines = f.readlines()
        lines = lines[lines.index('[\n'):]
    with open(FILENAME_FIXED, "w") as f:
        f.writelines(lines)
    with open(FILENAME_FIXED, "r") as f:
        data = json.load(f)
        for d in data:
            value = d.get("fields", {}).get("value")
            if value is not None and not isinstance(value, str):
                d["fields"]["value"] = str(value)
    with open(FILENAME_FIXED, "w") as f:
        f.write(json.dumps(data, indent=4, ensure_ascii=False))
    return data


if __name__ == "__main__":
    restore()

I also had to remove every sentry.useremail block from the backup file.

Error while importing to an empty database:

django.db.utils.IntegrityError: UniqueViolation('duplicate key value violates unique constraint "sentry_useremail_user_id_email_ade975f1_uniq"\nDETAIL:  Key (user_id, email)=(3, test@example.com) already exists.\n')
SQL: UPDATE "sentry_useremail" SET "user_id" = %s, "email" = %s, "validation_hash" = %s, "date_hash_added" = %s, "is_verified" = %s WHERE "sentry_useremail"."id" = %s

Email addresses are probably already added by the sentry.user backups...

This script caused us issues with boolean values being converted to uppercase string "False". The import didn't like them.

I've adapted the script to account for this:

import json

FILENAME = "backup.json"
FILENAME_FIXED = "backup_fixed_2.json"


def restore():
    with open(FILENAME, "r") as f:
        lines = f.readlines()
        lines = lines[lines.index('[\n'):]
    with open(FILENAME_FIXED, "w") as f:
        f.writelines(lines)
    with open(FILENAME_FIXED, "r") as f:
        data = json.load(f)
        for d in data:
            value = d.get("fields", {}).get("value")
            if isinstance(value, bool):
                d["fields"]["value"] = str(value).lower()
            elif value is not None and not isinstance(value, str):
                d["fields"]["value"] = str(value)
    with open(FILENAME_FIXED, "w") as f:
        f.write(json.dumps(data, indent=4, ensure_ascii=False))
    return data


if __name__ == "__main__":
    restore()

This script is broken and imports booleans as (encoded) strings into the database.
A True should be encoded as "gAKILg==" but gets inserted as "gAJYBAAAAFRydWVxAC4=" into the database.
After hours of debugging i finally managed to fix my installation with multiple manual db queries.
Sadly i dont have every query in my history, but some are here:

update sentry_option set value = 'gAKILg==' where value = 'gAJYBAAAAFRydWVxAC4=';
update sentry_organizationoptions set value = 'gAKILg==' where value = 'gAJYBAAAAFRydWVxAC4=';
update sentry_projectoptions set value = 'gAKILg==' where value = 'gAJYBAAAAFRydWVxAC4=';

@Lucaber , my adapted script above should be of help if you have your original exported backup.

If i remember correctly, numbers were also saved incorrectly as base64 encoded strings with quotes. 8 -> "8" | base64.
This resulted in broken "sentry:option-epoch" options.