2020-12-24 14:13:19 web_php 2206 ERR Invalid skin css 'flat' setting to base /usr/share/zoneminder/www/index.php 116
The line it references is the error message, which really doesnt help much. I have searched and there are several threads about this issue, but they all seem to be much older versions than 1.34
I have selected every possible option display and config settingsd related to CSS, and saved and restarted, same thing, log is filled with the flat error.
I believe I discovered the cause of this error. The "flat" CSS is read from a cookie. If you carry over a cookie from a previous version of ZM, you get this error, apparently there is some stuff in the cookie that wont work with the new version.
Deleting all the cookies for the site eliminated the nonstop stream of errors in the log.
This might be a worthwhile addition to the upgrade instructions, to delete all cookies for the site in your browser during the upgrade. Or perhaps, if this error is encountered, force deletion and reset of the cookie in the code.