You are right, probably caused by extra buttons added.
But if you backup the index.template.php from the default theme and save it on your local PC, then proceed with the install even if you have an error from that file.
Then you can download the premade manual zip file from the install board sticky post, and extract the zip, take the index.template.php from that file and replace with the broken one.
Refresh and TP are there.
Then open your saved index.template.php (backup you made) and take the buttons code out from it and place it in the new file you just uploaded, then you will have the bittons in the new file, and TP installed