Internal Server Error

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Hello concrete5 gurus from a relative beginner. Would appreciate some help with a sudden problem.
I have a similar problem as was described in the following:
https://www.concrete5.org/community/forums/installation/internal-ser...
I'm running 5.7.5.6 with fruitful(1.2) and PHP 5.4.43
Everything was working fine until last Monday when suddenly several pages developed an HTTP 500 error. The index page and several sub-pages work well, but about three pages developed the symptom at the same time. I tried the suggestions from the link above: Pretty URLs were never turned on and I really didn't understand the change to .htaccess which is here:
AddHandler application/x-httpd-php54 .php
rewriteengine off

I had the backup files from before the problem restored, but still no luck.
I would really like to fix this if possible. One page in particular would be difficult to re-create.
Don't know what else to tell you at this point.
Any help would be appreciated.
Gary