only clear backup cache if specific fields changed

src/settings.js

@@ -416,7 +416,11 @@ function setBackupConfig(backupConfig, callback) {
                 delete backupConfig.password;
             }
 
-            backups.cleanupCacheFilesSync();
+            // if any of these changes, we have to clear the cache
+            if ([ 'format', 'provider', 'prefix', 'bucket', 'region', 'endpoint', 'backupFolder', 'mountPoint', 'encryption' ].some(p => backupConfig[p] !== currentConfig[p])) {
+                debug('setBackupConfig: clearing backup cache');
+                backups.cleanupCacheFilesSync();
+            }
 
             settingsdb.set(exports.BACKUP_CONFIG_KEY, JSON.stringify(backupConfig), function (error) {
                 if (error) return callback(error);