Although my website was functioning properly on Monday, 12/11/17, it exhibited several errors the following day (12/12/17). No changes were made to any of the files after 12/6 (the last being an SSI text file on 12/6). Several of the base html files are nearly nine years old (I modify the SSI include files as needed, and rarely touch the others).
The site is coded in XHTML 1.0 Strict and until 12/12 the code has been fully compliant (per W3C HTML and CSS validators). I suspect it may have something to do with SSI, which is used extensively on the site, but I have seen only one possibly-related post concerning a recent Apache release (Critical Changes In Apache 2.4 - https://documentation.cpanel.net/display/EA/Critical+Changes+In+Apache+2.4#CriticalChangesInApache2.4-Server-SideInclude(SSI)ExpressionParser).
The server OS is LiteSpeed (Apache Version 2.4.29). The hosting company stated that no patches were applied to the system after the 11th, but I note that LiteSpeed released LSWS v. 5.2.3 on 12/11/17, the last day on which the website was working correctly. (This query is also posted on a LiteSpeed forum.)
All suggestions are welcome.