For some reason, the issue has reappeared on the same PC.
I have just tried two dozen fonts and none of them display the special characters mentioned above. The only font that displays the special characters is Segoe UI 
These are the entries under "Fonts" in the prefs.oxc file:
<fonts>
<files>Segoe UI/-24,400,0,0,0,1,0,200,9</files>
<tree>Segoe UI/-24,400,0,0,0,1,0,200,9</tree>
<status>Segoe UI/-24,400,0,0,0,1,0,200,9</status>
<header>Segoe UI/-24,400,0,0,0,1,0,200,9</header>
<thumbnails>Segoe UI/-24,400,0,0,0,1,0,200,9</thumbnails>
<foldertabs>Segoe UI/-24,400,0,0,0,1,0,200,9</foldertabs>
<fdb>Segoe UI/-24,600,0,0,0,1,0,200,9</fdb>
<log>Courier New/-24,400,0,0,0,1,0,200,9</log>
<groups>Segoe UI/-29,400,0,0,0,1,0,200,11</groups>
<panelnc>Segoe UI/-24,400,0,0,0,1,0,200,9</panelnc>
<jobsbar>Segoe UI/-24,400,0,0,0,1,0,200,9</jobsbar>
<metapane>Segoe UI/-24,400,0,0,0,1,0,200,9</metapane>
<renamemacro>Consolas/-24,400,0,0,0,1,0,200,9</renamemacro>
<infotips>Segoe UI/-24,400,0,0,0,1,0,200,9</infotips>
<code>Consolas/-24,400,0,0,0,1,0,200,9</code>
</fonts>