The phrase "serious backlog" means a situation in which there is a large and growing amount of work that needs to be completed, and it is causing delays and problems. Full definition
Suppliers are delivering again now, but there's a serious backlog and some storage terminals are very low indeed. (mummymummymum.com)
The Provincial Division and the Ministry of the Attorney General had known for years that a serious backlog of criminal cases was building. (ontariocourts.ca)
There is a serious backlog of family law cases in Ontario courts and this often results in long delays. (familylawhelp.ca)