LONDON — Traders who have been frantically following market swerves, tied to the trade war and a broad economic slowdown, had an unexpected respite on Friday.

The London Stock Exchange did not move. For about an hour and 40 minutes.

One of the world’s oldest exchanges was unable to handle trades at its scheduled open time, and administrators scrabbled to reassure investors. Trading should have started at 8 a.m., but the exchange was not fully operational until 9:40 a.m. because of a “technical software issue,” it said in a statement.

The exchange said it could not provide a comprehensive list of securities that were affected, but added that trading was delayed on two important indexes, the FTSE 100 and the FTSE 250, as well as some other securities. Shares in the London Stock Exchange Group were up 1.5 percent by late morning.

On its system notice board, the exchange offered advice that will be familiar — or perhaps maddening — to anyone who has suffered from I.T. issues before: Log out and log back in again.