What version of Ubuntu MATE did you install? 18.04 LTS or 18.10 LTS? If you let me know that and what kernel version you are running (in case you've installed any updates) I'll try it on mine and see if I can determine what might be causing this.
Also, try running the following to manually load the i2c kernel module and let me know if you get any errors:
sudo modprobe i2c-gpio-custom bus0=50,476,480
If you run the above modprobe command and receive no errors have a look and see if the /dev/i2c-50 character device now exits.
The output of: ls -ld /dev/i2c-50
should look like: crw-rw---- 1 root i2c 89, 50 Mar 12 19:59 /dev/i2c-50
If the modprobe command runs without errors and the /dev/i2c-50 character file exits then the bn055 examples 'should' work.
If you are receiving the error about the file /dev/i2c-50 not being present then there is an issue with the i2c-gpio-custom.service either not starting at all or erroring-out when trying to start. The service basically runs the modprobe command above to insert the kernel module when you run: sudo systemctl start i2c-gpio-custom
and then modprobe -r when you run: sudo systemctl stop i2c-gpio-custom
You can also try manually running the systemctl start/stop commands and then use: dmesg to display the kernel message buffer right after attempting to start/stop the i2c-gpio-custom service and get an idea of what might be going on. If the driver is loading you will see a message entry at the bottom of your dmesg output similar to:
[265641.299237] PKCS#7 signature not signed with a trusted key
[265641.299821] Custom GPIO-based I2C driver version 0.1.1
[265641.300289] i2c-gpio i2c-gpio.50: using lines 476 (SDA) and 480 (SCL)
Let me know how the above goes and if there are still errors I'll try installing the same Ubuntu version (and kernel version) on mine and see if I can determine what the issue is and a workaround for it.