Hello @nchaveiro and @TrystanLea

I made again a complete clean install on a spare Pi with the following information:

Server Information
Services
emonhub Active Running
emoncms_mqtt Active Running
feedwriter Active Running - sleep 60s
service-runner Active Running
emonPiLCD Active Exited
redis-server Active Running
mosquitto Active Running
Emoncms Version low-write 9.9.8
Modules Administration : App v1.2.1 : Backup v1.1.6 : EmonHub Config v1.1.0 : Dashboard v1.3.3 : Device v1.2.1 : EventProcesses : Feed : Graph v1.2.3 : Input : Postprocess v1.0.0 : CoreProcess : Schedule : Network Setup v1.0.0 : sync : Time : User : Visualisation : WiFi v1.3.1
Git URL: GitHub - emoncms/emoncms: Web-app for processing, logging and visualising energy, temperature and other environmental data : Branch: * stable : Describe: v9.5.1-1775-gd0db7a57
Server OS Linux 4.14.71-v7+
Host emonpi : emonpi : (192.168.2.241)
Date 2019-04-07 08:06:30 UTC
Uptime 08:06:30 up 21 min, 0 users, load average: 0.08, 0.05, 0.03
HTTP Server Apache/2.4.25 (Raspbian) HTTP/1.1 CGI/1.1 80
MySQL Version 5.5.5-10.1.23-MariaDB-9+deb9u1
Host 127.0.0.1 (127.0.0.1)
Date 2019-04-07 08:06:29 (UTC 00:00‌​)
Stats Uptime: 34289 Threads: 3 Questions: 550 Slow queries: 0 Opens: 24 Flush tables: 1 Open tables: 18 Queries per second avg: 0.016
Redis Version 3.2.6
Host localhost:6379 (127.0.0.1)
Size
Uptime 0 days
MQTT Server Version Mosquitto 1.4.10
Host localhost:1883 (127.0.0.1)
Pi Model Raspberry Pi 2 Model B Rev 1.1 - 1 GB (Sony UK)
SoC Broadcom BCM2835
Serial num. 67A92B68
Temperature CPU: 34.70°C - GPU: 34.7’C
Release emonSD-30Oct18
Memory RAM Used: 14.81% Total: 976.74 MB Used: 144.66 MB Free: 832.08 MB
Swap Used: 0.00% Total: 100 MB Used: 0 B Free: 100 MB
Disk Mount Stats
/ Used: 39.87% Total: 3.81 GB Used: 1.52 GB Free: 2.11 GB
/boot Used: 51.69% Total: 42.52 MB Used: 21.98 MB Free: 20.54 MB
/home/pi/data Used: 3.44% Total: 3.21 GB Used: 112.81 MB Free: 2.93 GB
PHP Version 7.0.30-0+deb9u1 (Zend Version 3.0.0)
Modules apache2handler : calendar v7.0.30-0+deb9u1 : Core v7.0.30-0+deb9u1 : ctype v7.0.30-0+deb9u1 : curl v7.0.30-0+deb9u1 : date v7.0.30-0+deb9u1 : dom v20031129 : exif v7.0.30-0+deb9u1 : fileinfo v1.0.5 : filter v7.0.30-0+deb9u1 : ftp v7.0.30-0+deb9u1 : gd v7.0.30-0+deb9u1 : gettext v7.0.30-0+deb9u1 : hash v1.0 : iconv v7.0.30-0+deb9u1 : igbinary v2.0.1 : json v1.4.0 : libxml v7.0.30-0+deb9u1 : mbstring v7.0.30-0+deb9u1 : mcrypt v7.0.30-0+deb9u1 : mosquitto v0.4.0 : mysqli v7.0.30-0+deb9u1 : mysqlnd vmysqlnd 5.0.12-dev - 20150407 - $Id: b5c5906d452ec590732a93b051f3827e02749b83 $ : openssl v7.0.30-0+deb9u1 : pcre v7.0.30-0+deb9u1 : PDO v7.0.30-0+deb9u1 : pdo_mysql v7.0.30-0+deb9u1 : Phar v2.0.2 : posix v7.0.30-0+deb9u1 : readline v7.0.30-0+deb9u1 : redis v4.1.1 : Reflection v7.0.30-0+deb9u1 : session v7.0.30-0+deb9u1 : shmop v7.0.30-0+deb9u1 : SimpleXML v7.0.30-0+deb9u1 : sockets v7.0.30-0+deb9u1 : SPL v7.0.30-0+deb9u1 : standard v7.0.30-0+deb9u1 : sysvmsg v7.0.30-0+deb9u1 : sysvsem v7.0.30-0+deb9u1 : sysvshm v7.0.30-0+deb9u1 : tokenizer v7.0.30-0+deb9u1 : wddx v7.0.30-0+deb9u1 : xml v7.0.30-0+deb9u1 : xmlreader v7.0.30-0+deb9u1 : xmlwriter v7.0.30-0+deb9u1 : xsl v7.0.30-0+deb9u1 : Zend OPcache v7.0.30-0+deb9u1 : zlib v7.0.30-0+deb9u1
Client Information
HTTP Browser Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:66.0) Gecko/20100101 Firefox/66.0
Screen Resolution 1525 x 858
Window Size 1506 x 730

Create the example standards physical inputs from API help, generate the the correspondent feeds. No customization made in the creation.
When try to create the virtual feed, with this feeds, the error occurs. I think in fact is a bug from the last update made by @TrystanLea, like you explain @nchaveiro.

Example of feeds add to make the virtual:

Apply the command:

It’s possible to correct this update @TrystanLea ?

Thanks for your attention and help.