(Reuters) - Customers of financial services firm Charles Schwab Corp faced difficulties in executing trades on Monday, according to outage monitoring website Downdetector.com. Downdetector showed that there were more than 3,300 incidents of people reporting issues. It was not immediately clear what caused the problems. (bit.ly/3nie4Vd) Charles Schwab said on Twitter that certain online functionalities may be slow or unavailable and that it was working to resolve the issue as quickly as possible. “Cannot trade right now ... very frustrating ... ‘invalid symbol’ when you try to execute,” one user said on Downdetector. Brokerages have faced a massive surge in trading volumes after the first successful data from a large-scale clinical trial of a coronavirus vaccine last month.