Ok guys. So I decided to do some research into the import data (made publicly available by the port administration!) at the Beirut Port. Government sources reported that the explosion was due to an ammonium nitrate shipment from six years ago that was not properly taken care...

Government sources reported a massive shipment of 2750 tons of ammonium nitrate were set off. I checked the entire record of imports and we actually find a perfect match. February 2014, 2750 tons of "Fertilizer" (i.e. likely ammonium nitrate) was brought into the port.

If you would like the direct link from where I got this screenshot, please see the following. You can directly access all the information from here: http://www.portdebeyrouth.com/index.php/en/members-area/old-statistics#

From the dataset we can also find that the cargo ship carrying the ammonium nitrate shipment was the "Al Mostafa" cargo ship operating under the Lebanese Shipping Union. The ship left the port of Beirut on the 25th of February 2014.

Finally, from multiple datasets, the port of origin of the shipment was the Damietta Port in Egypt. Some more relevant information for those doing some research:
ship IMO: 7800136
website of the ship owner: https://www.almustafashipping.com/ 

And of course, this doesn't concretely prove anything. However, I think it provides strong corroboration of the government narrative/explanation, as the data is consistent. Of course, this could be the government using the data to make a false statement, so we need more info...

..specifically regarding whether the original shipment was ever moved from the storage site (i.e. Warehouse 12 of the port), and I think our best bet here is to scour social media to try and find some picture taken between 2014 and 2020 showing the chemicals in storage

I think this thread is a very good start: https://twitter.com/FirasMaksad/status/1290864772355850241?s=20