503 Bosch motor error is implied for the speed sensor error category.
503 is the on-screen error for a speed sensor issue. If you are able to get the bike and connect to the bosch diagnostic software, then you will get more detail on the error type.
The first possibility of this error is a pretty common one - the magnet location. As there is a magnet that resolves the signal outputting to the sensor and delivering the data about your ebike speed. One of the issues off this is misalignment of the sensor or the magnet, it could be physical damage after strong kinetic force like hitting rocks or a tree sideways with your rear wheel, magnet and the sensor is quite sensitive parts of your ebike so any kind of heavier or visible damage might easily bring up the error to your screen.
Furthermore there is always the possibility of dirt - any kind of big concentration of dirt and residue on the speed sensor parts can induce the bad frequency of signal delivery to the sensor from the magnet.
Good idea is to check if you have the original Bosch magnet that provides the most durability out of all other third party magnets that are available in the market. So do make sure that you have an original magnet.
Most common of this error is the issue of water, sludge or any other residue buildup in the sensor - motor areas. This is not an expensive part to change so the repairs are necessary, furthermore keep in mind that your ebike always requires the same level of care like any other travel gear. So don’t forget to use the best products suited for your ebike care and protection. Bike chains and all the other parts that move very easily require constant lubrication and cleaning, in case of dirt buildup and no care for these parts, they can easily break and damage other vital parts of your e-bike!