Theme customizer menu doesn't show up -- Concrete5-8.5.4 + Framework Theme-1.7.7

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Hi,
When I try to edit page 'Design'--> 'Theme customize', the theme customize menu doesn't show up and remains empty.

The browser (Edge, Chrome and Firefox) console log is always like below(from Edge):

GEThttps://xxxxxx.fi/ccm/system/panels/page/design/customize/2?cID=1... 500 (Internal Server Error)
send @ jquery.js?ccm_nocache=d846f33d9148b37e63b22ff9a7339155be77a7f9:4
ajax @ jquery.js?ccm_nocache=d846f33d9148b37e63b22ff9a7339155be77a7f9:4
n.fn.load @ jquery.js?ccm_nocache=d846f33d9148b37e63b22ff9a7339155be77a7f9:4
(anonymous) @ app.js?ccm_nocache=d846f33d9148b37e63b22ff9a7339155be77a7f9:3
dispatch @ jquery.js?ccm_nocache=d846f33d9148b37e63b22ff9a7339155be77a7f9:3
r.handle @ jquery.js?ccm_nocache=d846f33d9148b37e63b22ff9a7339155be77a7f9:3

Few more hints:
1. This is brand new installation of concrete5 - 8.5.4 + Framework Theme-1.7.7. No modifications done to the pages.

2. When I switch back to default Elemental Theme the Design/Customization menu works like charm. But anytime I switch to Framework, it stops working

3. I have the pretty url 'hide index.php' enabled but have tried also 'hide index.php' disabled, that doesn't have any effect


Status: New

concrete5 Environment Information

# concrete5 Version
Core Version - 8.5.4
Version Installed - 8.5.4
Database Version - 20200609145307

# Database Information
Version: 5.7.30-log
SQL Mode: ONLY_FULL_GROUP_BY,STRICT_TRANS_TABLES,NO_ZERO_IN_DATE,NO_ZERO_DATE,ERROR_FOR_DIVISION_BY_ZERO,NO_AUTO_CREATE_USER,NO_ENGINE_SUBSTITUTION

# concrete5 Packages
Framework Theme (1.7.7)

# concrete5 Overrides
None

# concrete5 Cache Settings
Block Cache - On
Overrides Cache - On
Full Page Caching - Off
Full Page Cache Lifetime - Every 6 hours (default setting).

# Server Software
Apache

# Server API
cgi-fcgi

# PHP Version
7.4.7

# PHP Extensions
bcmath, bz2, calendar, cgi-fcgi, Core, ctype, curl, date, dba, dom, enchant, exif, fileinfo, filter, ftp, gd, gettext, gmp, hash, iconv, imap, intl, json, ldap, libxml, lzf, mbstring, mysqli, mysqlnd, openssl, pcntl, pcre, PDFlib, PDO, pdo_mysql, PDO_ODBC, pdo_pgsql, pdo_sqlite, pgsql, Phar, posix, pspell, readline, Reflection, session, shmop, SimpleXML, snmp, soap, sockets, SPL, sqlite3, standard, sysvmsg, sysvsem, sysvshm, tokenizer, xml, xmlreader, xmlrpc, xmlwriter, xsl, zip, zlib

# PHP Settings
max_execution_time - 30
log_errors_max_len - 1024
max_file_uploads - 20
max_input_nesting_level - 64
max_input_time - -1
max_input_vars - 1000
memory_limit - 512M
post_max_size - 16M
upload_max_filesize - 16M
ldap.max_links - Unlimited
mbstring.regex_retry_limit - 1000000
mbstring.regex_stack_limit - 100000
mysqli.max_links - Unlimited
mysqli.max_persistent - Unlimited
pcre.backtrack_limit - 1000000
pcre.recursion_limit - 100000
pgsql.max_links - Unlimited
pgsql.max_persistent - Unlimited
session.cache_limiter - <i>no value</i>
session.gc_maxlifetime - 7200
soap.wsdl_cache_limit - 5
unserialize_max_depth - 4096

Browser User-Agent String

Firefox 77.0.1 (64-bit), Google Chrome 83.0.4103.116 (Official Build) (64-bit), MS Edge 83.0.478.56 (Official build) (64-bit)