"Sometimes and limited problem to a single PC", makes me think that this could be related to the keyboard, whether the num-pad is enabled or not or Ohh instead of a zero entered Would really be interested to see a screenshot from both PCs with the entered material number and the message at the same screen together with the time and date from the PC. Actually one PC should be able to get in, so it should be a screenshot from the first screen to be extended along with the PC time.
The time is needed to validate whether the material really existed