I have only been a Greeter for 2 months, so I don't have as much history of volunteering issues as the rest of the fantastic Greeter team. The 2 biggest problems I have seen is email confirmation and as Carole has said, Tag issues. The email issues are usually that the link isn't clickable or it leads to a blank page. Both of which is usually solved by copy/paste or adjusting email/browser settings.
The Tag issues I have encountered are as Carole has said with the system not accepting the entries, people being confused as to the essence of the tag and also language translations. Tag in English equates to label in this sense but in German it means Day, and I have been asked on 2 or 3 occassions if a date needs to be put in and if so whether it is an ancestral date or todays date. Although technically it shouldn't stop people volunteering, it makes for frustration and puts people off joining.
I will ask for further information in future.