There are so many potential causes for this error. However, it is common for that error to happen when you have an abnormally large amount of deposition in a cross section. This can occur if that cross section is much larger than adjacent cross sections, if ineffective flow areas and mobile bed limits are not properly defined, or if there is just something preventing sediment from passing through that cross section. Check your cross sections. Make sure you don't have a sudden decrease in velocity where the unrealistic change is happening. If so, you may need to better define contraction and expansion through that reach by including and/or adjusting your ineffective flow areas.
i have the same problem now
i have checked allready width of all cross sections and they are almost the same
i have a problem at one certain cross section, but when i have done all this changes in this cross section the sediment analisys still doesn't work.