Invalid UTF-8 sequence in argument

Backwpup show me a Warning and do not backup folders anymore.

I don’t know if it happens since last backwpup update, but I haven’t changed any settings for a long time

Can you help me to resolve this?

Following is the log of the warning:

[INFO] BackWPup version 3.1.1; A project of Inpsyde GmbH
[INFO] WordPress version 3.8
[INFO] Blog url: https://xxxx
[INFO] BackWPup job: Full Backup; DBCHECK+DBDUMP+DBOPTIMIZE+FILE
[INFO] BackWPup cron: 0 5 * * 1; Next: lun, 13 gen 2014 @ 05:00
[INFO] BackWPup job started manually
[INFO] PHP ver.: 5.2.17; cgi-fcgi; Linux
[INFO] Maximum PHP script execution time is 120 seconds
[INFO] MySQL ver.: 5.0.92-enterprise-gpl-log
[INFO] curl ver.: 7.19.7; NSS/3.14.0.0
[INFO] Temp folder is: /web/xxxx
[INFO] Logfile is: /web/xxx/www.xxx.it/logs/backwpup_log_fec0ab_2014-01-08_14-40-56.html
[INFO] Backup type is: archive
[INFO] Backup file is: /web/www.xxxx.it/backup/backup_Full_2014-01-08_14-40-57.tar.gz
[08-gen-2014 14:40:57] 1. Try to backup database …
[08-gen-2014 14:40:57] Connected to database Sqlxxxx_1 on xx.yy.zz.178
[08-gen-2014 14:40:57] Backup database table "wp_commentmeta" with "3144" records
[08-gen-2014 14:40:57] Backup database table "wp_comments" with "353" records
...
...
...
[08-gen-2014 14:41:01] Backup database table "wp_xmlgooglemaps_gpxfile_item" with "0" records
[08-gen-2014 14:41:03] Added database dump "Sqlxxxx_1.sql" with 12,83 MB to backup file list
[08-gen-2014 14:41:03] Database backup done!
[08-gen-2014 14:41:03] 1. Trying to make a list of folders to back up …
[08-gen-2014 14:41:40] WARNING: json_encode(): Invalid UTF-8 sequence in argument
[08-gen-2014 14:41:40] 1521 folders to backup.
[08-gen-2014 14:41:40] 1. Trying to generate a manifest file …
[08-gen-2014 14:41:40] Added manifest.json file with 6,92 kB to backup file list.
[08-gen-2014 14:41:40] 1. Trying to create backup archive …
[08-gen-2014 14:41:40] Compressing files as TarGz. Please be patient, this may take a moment.
[08-gen-2014 14:41:43] Backup archive created.
[08-gen-2014 14:41:43] Archive size is 2,12 MB.
[08-gen-2014 14:41:43] 6 Files with 226,00 B in Archive.
...
...
...
[08-gen-2014 14:41:44] WARNING: Job finished with warnings in 48 seconds. Please resolve them for correct execution.

Thanks in advance

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