[horde] Horde IMP Quota dovecot errors

[Admin Beckspaced]

Dear Horde users,

I have installed Horde (horde) 6.0.0alpha6 with Webmail (imp) 7.0.0alpha13
install was done via instructions at 
https://github.com/horde/horde-deployment

all is working fine so far but I also wanted to enable the IMAP quota
it was working fine with horde version 5.x imp version 6.x running php 
version 7.4

in /imp/config/backends.local.php quota is enabled as follows

// IMAP server
$servers['imap'] = array(
     // ENABLED by default; will connect to IMAP port on local server
     'disabled' => false,
     'name' => 'IMAP Server',
     'hostspec' => 'localhost',
     ...
     'quota' => array(
         'driver' => 'imap',
         'params' => array(
             'hide_when_unlimited' => true,
             'unit' => 'MB'
         )
     ),
);

Browsing to webmail in horde the following error messages appear:

Mar 07 09:08:14 cx20 dovecot[388]: imap-login: Login: 
user=<admin at beckspaced.com>, method=PLAIN, rip=127.0.0.1, lip=127.0.0.1, 
mpid=3350, TLS, session=<akfU6Ur2btZ/AAAB>
Mar 07 09:08:14 cx20 HORDE[2530]: [imp] TypeError: round(): Argument #1 
($num) must be of type int|float, string given in 
/srv/www/horde/httpdocs/vendor/horde/imp/lib/Ajax/Queue.php:293
Mar 07 09:08:14 cx20 HORDE[2530]: Stack trace:
Mar 07 09:08:14 cx20 HORDE[2530]: #0 
/srv/www/horde/httpdocs/vendor/horde/imp/lib/Ajax/Queue.php(293): round()
Mar 07 09:08:14 cx20 HORDE[2530]: #1 
/srv/www/horde/httpdocs/vendor/horde/imp/lib/Ajax/Application.php(145): 
IMP_Ajax_Queue->add()
Mar 07 09:08:14 cx20 HORDE[2530]: #2 
/srv/www/horde/httpdocs/vendor/horde/imp/lib/Ajax/Application.php(134): 
IMP_Ajax_Application->getTasks()
Mar 07 09:08:14 cx20 HORDE[2530]: #3 
/srv/www/horde/httpdocs/vendor/horde/horde/services/ajax.php(70): 
IMP_Ajax_Application->send()
Mar 07 09:08:14 cx20 HORDE[2530]: #4 {main} [pid 2530 on line 74 of 
"/srv/www/horde/httpdocs/vendor/horde/core/lib/Horde/ErrorHandler.php"]
Mar 07 09:08:14 cx20 dovecot[388]: 
imap(admin at beckspaced.com)<3350><akfU6Ur2btZ/AAAB>: Disconnected: Logged 
out in=187 out=1161 deleted=0 expunged=0 trashed=0 hdr_count=0 
hdr_bytes=0 body_count=0 body_bytes=0

is this error fixable?

I'm currently running php 8.0.28 on an opensuse box leap 15.4

thanks
& greetings
Ralf