VQplus daemon won't start

Starting callback daemon fails immediately:

[root@freepbx asterisk]# fwconsole pm2 --list
+--------------+-------+---------+----------+------------------------+-----+----------+
| Process Name | PID   | Status  | Restarts | Uptime                 | CPU | Mem      |
+--------------+-------+---------+----------+------------------------+-----+----------+
| qcallback    | 0     | errored | 15       | 0                      | 0%  | 0.00B    |
+--------------+-------+---------+----------+------------------------+-----+----------+

There is nothing in qcallback_err.log, but the following shows up in qcallback_out.log:

2019-01-22 21:15 +01:00: 1 We have free SlOTS in the queue 41
2019-01-22 21:15 +01:00: libphonenumber\NumberParseException: Could not interpret numbers after plus-sign. in file /var/www/html/admin/modules/vqplus/vendor/giggsey/libphonenumber-for-php/src/PhoneNumberUtil.php on line 1616
Stack trace:
  1. libphonenumber\NumberParseException->() /var/www/html/admin/modules/vqplus/vendor/giggsey/libphonenumber-for-php/src/PhoneNumberUtil.php:1616
  2. libphonenumber\PhoneNumberUtil->parseHelper() /var/www/html/admin/modules/vqplus/vendor/giggsey/libphonenumber-for-php/src/PhoneNumberUtil.php:3015
  3. libphonenumber\PhoneNumberUtil->parse() /var/www/html/admin/modules/vqplus/Vqplus.class.php:28
  4. FreePBX\modules\Vqplus->checknumbertimezone() /var/www/html/admin/modules/vqplus/functions.inc/geters_seters.php:128
  5. vqplus_checknumbertimezone() /var/www/html/admin/modules/vqplus/functions.inc/callback-daemon.php:464
  6. vpos_check_callbacks() /var/www/html/admin/modules/vqplus/functions.inc/callback-daemon.php:598
  7. vqplus_callback_calls_load() /var/www/html/admin/modules/vqplus/functions.inc/callback-daemon.php:127
  8. include() /var/www/html/admin/modules/vqplus/queue-callback.php:25

There is a single pending callback showing, and the callback number does not have a + character. I don't know what's triggering the "Could not interpret numbers after plus-sign" error.

MariaDB [asterisk]> select * from vqplus_callback_calls;
+-----------+----------------+------+----------+-------------+------------------+
| queue_num | uniqueid       | cbid | position | callback    | queue_entry_time |
+-----------+----------------+------+----------+-------------+------------------+
| 41        | 1548058898.599 |    1 |        1 | <redacted>  | 1548058955       |
+-----------+----------------+------+----------+-------------+------------------+
1 row in set (0.00 sec)