apache2 error after upgrade Buster->Bookworm

this email may go anywhere.
After putting it off for ages I upgraded the PC that does Dovecot, Roundcube and some other things from Buster to Bookworm.
There seems to be an error with apache2.
The apache2 index file I made as a page of links to things on the server.
Likely you are not supposed to do it that way.
A browser now offers to download this index.html instead of previously it being presented by apache2.
So now the link to the roundcube webmail doesn't work. A browser offers to download it.
Getmail, Dovecot seems to be getting the emails and putting them in the correct Maildir directories.

although
$ apachectl configtest
Syntax OK

$ apache2 -V
[Wed Oct 23 08:57:39.760030 2024] [core:warn] [pid 4112:tid 4112] AH00111: Config variable ${APACHE_RUN_DIR} is not defined
apache2: Syntax error on line 80 of /etc/apache2/apache2.conf: DefaultRuntimeDir must be a valid directory, absolute or relative to ServerRoot
Server version: Apache/2.4.62 (Debian)

Line 80 is DefaultRuntimeDir ${APACHE_RUN_DIR}

uncommenting line 69 ServerRoot "/etc/apache2"
and typing "source /etc/apache2/envvars" seems to have made "Config variable ${APACHE_RUN_DIR} is not defined" go away.
I know I shouldn't type things without knowing what they do but source has no manpage.

Likely it is something in the configuration files in /etc/apache2.
I wonder if there is something in the configuration affected by the usrmerge.
Anybody had issues with apache2 upgrading from Buster to Bookworm?
mick