The error messages in error_log are the blacklisted ffmpeg filters that do not work and which should NOT even load.
The file in cinelerra-5.1/ffmpeg/plugin.opts contains the non-working filters, preceded by a # sign (for example, aap or acrossfade, etc). That file is copied to cinelerra-5.1/bin/ffmpeg/plugin.opts in the build. If it is not there, then the build must have failed.
Can you check your build log for the word "Error" -- file is named "log" in the cinelerra-5.1 directory. Not sure how you ran the build but for now, could you do the build again using the cinelerra-5.1/bld.sh file if that is not what you used? which records the building on a file in that same directory, called "log".