TL;DR: The `.terragrunt-cache` holds a backend record written by an older version that the current binary can't decode. The error suggests `-reconfigure`, but the real fix is deleting the stale `.terragrunt-cache` so init starts clean.

```text
Error: Failed to decode current backend config

The backend configuration created by the most recent run of "terraform init"
could not be decoded: unsupported attribute "lock_table". The configuration
may have been initialized by an earlier version that used an incompatible
configuration structure. Run "terraform init -reconfigure" to force
re-initialization of the backend.
```

## Steps

1. Delete the unit's cache: `rm -rf .terragrunt-cache` (from the unit directory).
   Expected: the stale backend record is gone.
2. Re-run `terragrunt init`.
   Expected: `Initializing the backend...` completes without the decode error.
3. If MANY units are affected (e.g. after a version bump across the repo): `find . -type d -name ".terragrunt-cache" -prune -exec rm -rf {} +`.
   Expected: all units re-init clean.

## When this applies

- `Failed to decode current backend config` / `unsupported attribute` right after upgrading the tofu/terraform version or the backend schema.
- The config itself didn't change; only the tool version did.

## When it doesn't apply

- You actually CHANGED the backend config (bucket, key, region): then `-reconfigure` vs `-migrate-state` is the real decision, not cache clearing.
- `Backend initialization required`: that's a missing init, not a stale one.

## Tool versions

All Terragrunt versions (cache layout unchanged across 1.x).

## Why it happens

Terragrunt copies the module into `.terragrunt-cache` and init records the backend config there. After a version upgrade, the cached record uses a schema the new binary can't decode, and init fails before it ever re-reads your current config.

## Edge cases

- There is no `terragrunt clear-cache` command (removed in the 1.0 redesign); deleting the directory yourself IS the supported operation.
- Set `TG_DOWNLOAD_DIR` to move the cache outside the working tree if stale caches keep biting in CI.