sudo yunohost app shell wallabag2
php -m
special characters in the password are a problem
The password includes special characters, but which the Github issue you pointed out lists as allowed.
It could also be that the currently running PHP Version does not have php_pdo enabled
sudo yunohost app shell wallabag2
php -m
outputs:
wallabag2@server:~$ php -m
[PHP Modules]
calendar
Core
ctype
date
dom
exif
FFI
fileinfo
filter
ftp
gettext
hash
iconv
json
libxml
openssl
pcntl
pcre
PDO
Phar
posix
readline
Reflection
session
shmop
SimpleXML
sockets
sodium
SPL
standard
sysvmsg
sysvsem
sysvshm
tokenizer
xml
xmlreader
xmlwriter
xsl
Zend OPcache
zlib
[Zend Modules]
Zend OPcache
Note that loosing the user data is not such a big deal for me here. There is not so much, and I was able to find it in the DB dump of the autobackup.
However, uninstalling and then reinstalling Wallabag from YNH catalog (not from backup) leads to the same issue. And I notice that it seems to be the same on the YNH demo instance…
Same here with 2.5.4~ynh8, except I run YNH 11.3.0.2, and pdo_mysql and pdo_sqlite are enabled.
Grand merci pour cette solution 
For me, there is no solution: I uninstalled Wallabag and reinstalled it again: “invalid credentials” error after login with the admin user stated at installation over yunohost.
Yunohost: 11.3.0.2
Wallabag: 2.5.4~ynh9
This issue has been the same for months (at least that’s my feeling). So I hope one day there will be an out-of-the-box solution. My data are lost anyway.
Didn’t you make a backup (I mean the automated one) before the upgrade ?
You can still install an older version, before this issue happened.
I believe this is the last one (prior to the issue): yunohost app install https://github.com/YunoHost-Apps/wallabag2_ynh/tree/088f79ee1b99138e26319231062a21965b6d23d1