Why do MSSQL DB queries end with the "Query timed out" error?

Question

SQL Server

Some background information:
MSSQL Server Express
Database size: 9 GB.
Number of users: 50 pcs.
The situation is as follows: users of the organization through a certain program work with the database, in fact it is the base of subscribers of the organization, with many tables and fields. After the 1C base of the same organization and the customer base were separated to different servers (rphost ate up a huge amount of memory and choked sql), zen came for a while. A week later, users of the database began to be thrown out with the error "request timed out". The load on the server is less than 50%. No background indexing, backups, etc. are performed during business hours. Set sp_whoisactive - queries are executed in seconds.
In sp_who displays all connected users, some multiple times marked "AWAITING COMMAND".
As a result, they wanted the best, reduced the load on the server in anticipation of an increase in performance. Freed up 50% of resources.
Tell me please.