mi rispondo da solo. Ho trovato questo su xda:
"For TB, there is a solution. You need to go the the Backup Folder Location choice in Preferences. Look for the "Storage Provider" option at the top of the dialogue box. Tap it and choose "DocumentProvider Storage."
Storage Provider should now show "External Storage."
Navigate to the parent directory of your TB backups and choose it.
You should now be able to select your TB backup directory. (The "Use the Current Folder" choice should be active.)
There is another solution that TB offers, copied and pasted below from Item #31 on TB's troubleshooting page:
"There is an elegant way of giving TitaniumBackup write access to the external sdcard, without having to forfeit security .... Since the time of Android 4.3, Google uses a very clean security-solution by giving apps that request write access to external SD, permission only to their own folder – not to the entire card! In order to get TitaniumBackup to work without compromising security, you can simply create a folder on the external sdcard, using the following folder-structure: /storage/extSdCard/Android/data/com.keramidas.TitaniumBackup/
If you already have existing backups, move them with a file-explorer from their current place to: /storage/extSdCard/Android/data/com.keramidas.TitaniumBackup/ Lastly, go to the preferences of TitaniumBackup, choose “Backup folder location” and choose the above specified folder."
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Ho provato il primo metodo, ora funziona.