ok - seems strange. Let's confirm a few things...
First - you are running a raspberry pi 3 and libreelec 7.0.2?
You have been able to run and web access tvheadend addon 4.0 but not 4.2 from the LE repository?
What happened when you deleted all the files in the \\LIBREELEC\Userdata\addon_data\service.tvheadend42\channel folder? (likely the 4.0 folder instead)
You have tried uninstalling all versions of the tvheadend addon and then through another computer deleted all tvheadend folders in the \\LIBREELEC\Userdata\addon_data folder?
One thing we can check is what is getting downloaded...it seems from the log that zap2xml is running and pulling data. From your windows computer, go to the following folder \\LIBREELEC\Userdata\addon_data\tools.module.zap2xml
You should see an xmltv.xml file there - open it (it's just a text document so use wordpad) and see if there is program listing in there. The program info will start after the channel listing data, which will be the first set of lines. A program listing will look something like this:
<programme start="20160704120000 -0700" stop="20160704130000 -0700" channel="I25019.labs.zap2it.com">
<title lang="en">Judge Mathis</title>
<desc lang="en">The defendant says he was raised in foster care because of his mother's drug addiction. TV-PG • HD • CC • Aired: March 21, 2016</desc>
<date>20160321</date>
<category lang="en">Reality</category>
<category lang="en">Series</category>
<icon src="http://images.zap2it.com/assets/p185493_b_h3_aa/judge-mathis.jpg" />
<episode-num system="onscreen">S17E105</episode-num>
<episode-num system="dd_progid">EP00324230.3055</episode-num>
<episode-num system="xmltv_ns">16.104.</episode-num>
<video>
<aspect>16:9</aspect>
<quality>HDTV</quality>
</video>
<previously-shown start="20160321000000" />
<subtitles type="teletext" />
<rating>
<value>TV-PG</value>
</rating>
</programme>
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If that info is not in the xmltv file - then we can target zap2xml. Otherwise we are targeting a tvheadend issue.
The tvheadend versions I have are from CVH's builds. Here is the last version of 4.2 that contained the working list of muxes:
Dropbox - service.tvheadend42-7.0.101-1928.zip
You try uninstalling all versions of tvh, deleting the tvh folders in the userdata folder, reboot and then install this addon. If you install and can't access the web interface let me know and we can target that issue.