Effectivement, j'ai installé cette version hors dépôt, du fait d'un problème de compatibilité avec php7.
Voilà le log d'apache, mais ça ne me dit pas grand chose.
[Thu Mar 05 06:13:03.376232 2020] [mpm_prefork:notice] [pid 1294] AH00163: Apache/2.4.29 (Ubuntu) configured -- resuming normal operations
[Thu Mar 05 06:13:03.376259 2020] [core:notice] [pid 1294] AH00094: Command line: '/usr/sbin/apache2'
[Thu Mar 05 14:35:08.441369 2020] [mpm_prefork:notice] [pid 1294] AH00169: caught SIGTERM, shutting down
[Thu Mar 05 17:04:10.706886 2020] [mpm_prefork:notice] [pid 1308] AH00163: Apache/2.4.29 (Ubuntu) configured -- resuming normal operations
[Thu Mar 05 17:04:10.718421 2020] [core:notice] [pid 1308] AH00094: Command line: '/usr/sbin/apache2'
[Thu Mar 05 17:19:35.576006 2020] [mpm_prefork:notice] [pid 1308] AH00169: caught SIGTERM, shutting down
[Thu Mar 05 17:22:38.141983 2020] [mpm_prefork:notice] [pid 2917] AH00163: Apache/2.4.29 (Ubuntu) configured -- resuming normal operations
[Thu Mar 05 17:22:38.142042 2020] [core:notice] [pid 2917] AH00094: Command line: '/usr/sbin/apache2'
[Thu Mar 05 17:23:28.478133 2020] [php7:error] [pid 2920] [client 127.0.0.1:41830] PHP Fatal error: Uncaught TypeError: ucfirst() expects parameter 1 to be string, null given in /usr/share/phpmyadmin/libraries/classes/Plugins/Schema/Pdf/Pdf.php:272\nStack trace:\n#0 /usr/share/phpmyadmin/libraries/classes/Plugins/Schema/Pdf/Pdf.php(272): ucfirst(NULL)\n#1 /usr/share/phpmyadmin/vendor/tecnickcom/tcpdf/tcpdf.php(3560): PhpMyAdmin\Plugins\Schema\Pdf\Pdf->Header()\n#2 /usr/share/phpmyadmin/vendor/tecnickcom/tcpdf/tcpdf.php(3227): TCPDF->setHeader()\n#3 /usr/share/phpmyadmin/vendor/tecnickcom/tcpdf/tcpdf.php(3139): TCPDF->startPage('L', '', false)\n#4 /usr/share/phpmyadmin/libraries/classes/Plugins/Schema/Pdf/PdfRelationSchema.php(493): TCPDF->AddPage('L')\n#5 /usr/share/phpmyadmin/libraries/classes/Plugins/Schema/Pdf/PdfRelationSchema.php(134): PhpMyAdmin\Plugins\Schema\Pdf\PdfRelationSchema->dataDictionaryDoc(Array)\n#6 /usr/share/phpmyadmin/libraries/classes/Plugins/Schema/SchemaPdf.php(129): PhpMyAdmin\Plugins\Schema\Pdf\PdfRelationSchema->__construct('lego')\n#7 /usr/share/phpmyadmin/libraries/classes/Export.php(122 in /usr/share/phpmyadmin/libraries/classes/Plugins/Schema/Pdf/Pdf.php on line 272