Can you post a public link to your simulation?

If I assume your sim is: https://easyeda.com/jerome.abela/Logic-b178367b7e0b48e2b35fbf1173f199e9 then it does not run because there are several serious errors in the schematic. 1) You have not selected simulatable parts as described in: https://easyeda.com/forum/topic/How_to_find_simulatable_parts_in_EasyEDA-1YgasK2kC 2) You have not given any of the resistors values. 3) You have incorrectly configured the voltage sources V1 to V3. Please see: https://easyeda.com/forum/topic/Selecting_Sources_in_EasyEDA-oASM4mEyQ for more information. If you select your parts from the `EasyEDA Libs` in the left hand Navigation panel and: (i) simply edit the transistor names to 2N2907 and 2N2222 as necessary; (ii) define all the resistors to be say 1k; (iii) Select, place and configure Voltage Source for V1 to V3 and not Voltage Source(EasyEDA); then your circuit will simulate fine. * We recommend that all our users read and play with the examples in the EasyEDA Simulation eBook here: https://docs.google.com/document/u/1/d/1OWZVVFRAe_2NW3WratpkA_SGuHa5AcRow5ZRfvcoVTU/pub before attempting to run simulations in EasyEDA.

Thanks Andy for this very detailed answer, I am going to learn a lot from it! I think it would be nice to surface a more detailed error message. In the current situation, one could easily have everything right but one single mistake on the whole schematics, and no hint about where the error is. Are there any plans for this?

When you hit an [internal server error][1] it usually means some parts of your web server is not configured correctly or the application is trying to do something and the server is failing to carry out the request due to a conflict or restriction. This error can only be resolved by fixes to the Web server software . It is up to the administrators of the Web server site to locate and analyse the logs which should give further information about the error. [1]: http://net-informations.com/q/mis/500.html